A packaging and nullability release. The NAudio meta-package is now a single net9.0 package that brings the Windows backends (WinMM and WASAPI) to plain netX.0 projects, so a net10.0 console app on Windows gets working MP3 decoding and playback rather than a cross-platform subset that threw NotSupportedException. In exchange it no longer pulls in NAudio.Asio, NAudio.Dmo or NAudio.WinForms, which must now be referenced directly if used. NAudio.WinMM, NAudio.Asio, NAudio.Dmo and NAudio.Vst3 likewise now target plain net9.0.

Every NAudio package is now compiled with nullable reference types enabled. This is a source-level change with no binary breaks, but projects with nullable enabled may see new warnings, and a number of members that used to throw NullReferenceException now throw ObjectDisposedException or InvalidOperationException instead.

The fixes centre on WasapiRecorder/WasapiCapture restarting cleanly after both successful and failed starts, buffer and handle lifetime bugs in the winmm MIDI backend, and frame-aligned seeking in MediaFoundationReader.

• Breaking: the NAudio meta-package is now a single net9.0 package that pulls in NAudio.Core, NAudio.Midi, NAudio.WinMM and NAudio.Wasapi on every target framework. A plain netX.0 project on Windows (e.g. a console app on net10.0) now gets Mp3FileReader, WaveOut, WasapiPlayer and an AudioFileReader that reads MP3, where previously it silently got a cross-platform subset that threw NotSupportedException (#1472). It no longer includes NAudio.Asio, NAudio.Dmo or NAudio.WinForms: add those packages if you use AsioOut/AsioDevice, DirectSoundOut or the DMO effects, or WaveOutWindow/WaveInWindow/the NAudio.Gui controls. AudioFileReader now chooses its Media Foundation/ACM paths at runtime, and off Windows its NotSupportedException messages point to NAudio.SoundFile. NAudio.Extras is now single-target net9.0, with AudioPlaybackEngine marked Windows-only (#1485)
• NAudio.WinMM, NAudio.Asio, NAudio.Dmo and NAudio.Vst3 now target plain net9.0 instead of net9.0-windows, matching NAudio.Wasapi. They remain Windows-only, enforced by [SupportedOSPlatform("windows")], but can now be referenced directly from a plain netX.0 project (calls show CA1416 warnings unless guarded or the project is marked Windows-only) and built on Linux/macOS without EnableWindowsTargeting. No change for projects targeting netX.0-windows (#1482)
• Nullable reference types in NAudio.Core - NAudio.Core is now compiled with nullable reference types enabled, so its public API carries nullability annotations (e.g. WaveFileReader.ReadNextSampleFrame and Mp3Frame.LoadFromStream return nullable types, and Mp3Frame.RawData is byte[]? when a frame is read without its data). This is a source-level change only: there are no binary breaks, but projects with nullable enabled may see new warnings, and overrides of NAudio.Core virtual members may need matching annotations. XingHeader.LoadXingHeader now returns null for a frame read without its data instead of throwing. Thanks to @Seb-stian (#1452)
• Nullable reference types in NAudio.Midi - NAudio.Midi is now also compiled with nullable reference types enabled (e.g. MidiInMessageEventArgs.MidiEvent and NoteOnEvent.OffEvent are nullable). Source-level change only; projects with nullable enabled may see new warnings. MidiEvent.ReadNextEvent now throws FormatException instead of NullReferenceException for running status with no previous event, and RawMetaEvent treats null data as empty (#825)
• Nullable reference types in NAudio.WinMM - NAudio.WinMM is now compiled with nullable reference types enabled (e.g. AcmDriver.FindByShortName and WaveCapabilitiesHelpers.GetNameFromGuid return nullable types, and AcmDriver.ShowFormatChooseDialog has nullable out parameters). Source-level change only; projects with nullable enabled may see new warnings. WaveOut.OutputWaveFormat before Init now throws InvalidOperationException, AcmStream members throw ObjectDisposedException after dispose, and GetNameFromGuid returns null instead of throwing when the registry key is missing, where each previously threw NullReferenceException (#825)
• Nullable reference types in NAudio.SoundFile and NAudio.Sampler - both packages are now compiled with nullable reference types enabled (e.g. the SoundFileTags fields, SoundFileWriterOptions.Tags and SoundFileException.Function are nullable, and the TryLoad methods on WaveSampleLoader / ISfzSampleLoader mark left as non-null on success and right as null for mono). Source-level change only; projects with nullable enabled may see new warnings, and custom ISfzSampleLoader implementations may need matching annotations (#825)
• Nullable reference types in NAudio.WinForms, NAudio.Alsa and NAudio.Extras - these packages are now compiled with nullable reference types enabled (e.g. WaveViewer.WaveStream, AlsaException.Function and the control/device events are nullable). Source-level change only; projects with nullable enabled may see new warnings. AlsaOut.OutputWaveFormat and WaveOutWindow.OutputWaveFormat now throw InvalidOperationException before Init (previously null and NullReferenceException respectively), and WaveOutWindow.Play before Init throws InvalidOperationException instead of NullReferenceException (#825)
• Nullable reference types in NAudio.Asio - NAudio.Asio is now compiled with nullable reference types enabled (e.g. the optional AsioPlaybackOptions.OutputChannels / AsioDuplexOptions.InputChannels, the AsioDevice/AsioOut events and the AsioDriverExt callbacks are nullable; required option members such as Source and Processor stay non-nullable). Source-level change only; projects with nullable enabled may see new warnings. AsioOut members now throw ObjectDisposedException after Dispose and AsioOut.OutputWaveFormat throws InvalidOperationException when no playback source is initialised, where they previously threw NullReferenceException or returned null (#825)
• Nullable reference types in NAudio.Dmo - NAudio.Dmo is now compiled with nullable reference types enabled (e.g. DirectSoundOut.PlaybackStopped and DirectSoundDeviceInfo.ModuleName are nullable). Source-level change only; projects with nullable enabled may see new warnings. DirectSoundOut.OutputWaveFormat now throws InvalidOperationException before Init instead of returning null, and the MediaObject wrappers, DmoResampler.MediaObject, WindowsMediaMp3Decoder.MediaObject and DmoOutputDataBuffer.MediaBuffer throw ObjectDisposedException after Dispose instead of returning null or throwing NullReferenceException (#825)
• Nullable reference types in NAudio.Wasapi (CoreAudioApi and MediaFoundation) - the NAudio.CoreAudioApi and NAudio.MediaFoundation types are now annotated for nullable reference types (e.g. AudioClient.TryGetAcousticEchoCancellationControl, MMDeviceEnumerator.TryGetDefaultAudioEndpoint, MfSourceReader.ReadSample, PropertyStoreProperty.Value and Part.AudioVolumeLevel/AudioMute/JackDescription are nullable); WasapiOut, WasapiCapture and the other top-level classes follow in a separate change. Source-level change only; projects with nullable enabled may see new warnings. The CoreAudio and Media Foundation COM wrappers now throw ObjectDisposedException when used after Dispose instead of NullReferenceException, AudioSessionManager.Sessions throws InvalidOperationException instead of returning null when sessions are unavailable, and AudioSessionControl.AudioMeterInformation / SimpleAudioVolume throw NotSupportedException instead of returning null if the session does not expose those interfaces (#825)
• Nullable reference types in NAudio.Wasapi (playback, capture and Media Foundation classes) - WasapiOut, WasapiCapture, WasapiPlayer, WasapiRecorder, their builders and MediaFoundationReader/StreamMediaFoundationReader/MediaFoundationEncoder are now annotated too, completing NAudio.Wasapi (e.g. WasapiPlayer/WasapiRecorder.DeviceId and DeviceFriendlyName, LowLatencyUnavailableReason, WasapiRecorder.AcousticEchoCancellationControl, WasapiPlayer.GetSupportedExclusiveFormat, MediaFoundationEncoder.SelectMediaType and the playback/recording events are nullable). Source-level change only; projects with nullable enabled may see new warnings. The WASAPI classes now throw ObjectDisposedException when used after Dispose, and report InvalidOperationException instead of NullReferenceException through PlaybackStopped when playback is started without Init (#825)
• Nullable reference types in NAudio.MacOS - NAudio.MacOS is now compiled with nullable reference types enabled (e.g. AudioSystemObject.DefaultInputDevice/DefaultOutputDevice, the ConvertUIDToDevice-style lookups, AudioObject.Owner, AudioDevice.Icon, ExtendedAudioFileReaderSettings.OutputFormat, ExtendedAudioFileWriterSettings.FileType/ProvidingFormat and the player, recorder and property-listener events are nullable). Source-level change only; projects with nullable enabled may see new warnings. Object-list properties such as AudioSystemObject.Devices now skip unknown object IDs instead of returning null entries, the parameterless CoreAudioPlayer/CoreAudioRecorder constructors throw InvalidOperationException instead of ArgumentNullException when there is no default device, and ExtendedAudioFileWriter captures ProvidingFormat when created rather than re-reading the settings object on every write (#825)
• Fixed WasapiRecorder and WasapiCapture failing with AUDCLNT_E_ALREADY_INITIALIZED when restarted after a start that failed late in initialization (e.g. no AEC reference control); the original error is now reported again (#1493)
• Fixed WasapiRecorder and WasapiCapture being left in the Starting state when initialization throws, so later StartRecording calls failed with "Already recording". (#1490)
• Fixed WasapiRecorder not able to re-start after successfully stopping the recorder. (#1479)
• Fixed several buffer and handle lifetime bugs in the winmm MIDI backend: MidiOut.SendBuffer no longer frees a sysex buffer while the driver may still own it, MidiIn now stops input before resetting it, and neither class calls into winmm with a handle it never opened. MidiOut.SendBuffer now blocks until the driver releases the buffer and throws MmException if the send fails (#1478)
• Fixed MediaFoundationReader reporting a Position that was not a whole number of frames after seeking a compressed file (e.g. MP3 or AAC), up to a frame before the requested position, and a Length that could include a partial frame. Seeks now round down to a frame boundary and land exactly on it (#1488)