The documentation for AVAudioEngineConfigurationChangeNotification states When the audio engine’s I/O unit observes a change to the audio input or output hardware’s channel count or sample rate, the audio engine stops, uninitializes itself, and issues this notification. A user of my framework has reported a crash during notification processing on iOS 26.4 when the main mixer node is disconnected from the output node in order to reestablish the connection with a different format. The failing precondition is com.apple.coreaudio.avfaudio: required condition is false: !IsRunning(). The report was observed on iPhone 16 / iOS 26.4.2, ARM64, TestFlight build. The backtrace contains: [Last Exception Backtrace] 3 AVFAudio AVAudioEngineGraph::_DisconnectInput AVAudioEngineGraph.mm:2728 4 AVFAudio -[AVAudioEngine disconnectNodeInput:bus:] AVAudioEngine.mm:155 5 SFB sfb::AudioPlayer::handleAudioEngineConfigurationChange AudioPlayer.mm:2247 [Thread 18 Crashed] 9 SFB sfb::AudioPlayer::handleAudioEngineConfigurationChange AudioPlayer.mm:2212 … 14 AVFAudio IOUnitConfigurationChanged Has the behavior for AVAudioEngineConfigurationChangeNotification changed in iOS 26.4? It's simple enough to call [engine_ stop] in the notification handler but the documentation states this shouldn't be necessary. I've not observed a similar crash on previous iOS versions.

Are the AudioObject APIs (such as AudioObjectGetPropertyData, AudioObjectSetPropertyData, etc.) thread-safe? Meaning, for the same AudioObjectID is it safe to do things like: Get a property in one thread while setting the same property in another thread Set the same property in two different threads Add and remove property listeners in different threads Put differently, is there any internal synchronization or mutex for this kind of usage or is the burden on the caller? I was unable to find any documentation either way which makes me think that the APIs are not thread-safe.