Got the same problem here. Stacktrace from 4.24.2:
Assertion failed: SoundWave && SoundWave->bProcedural [File:D:/Build/++UE4/Sync/Engine/Source/Runtime/AudioMixer/Private/AudioMixerSourceBuffer.cpp] [Line: 507]
UE4Editor_Core!AssertFailedImplV() [d:\build\++ue4\sync\engine\source\runtime\core\private\misc\assertionmacros.cpp:105]
UE4Editor_Core!FDebug::CheckVerifyFailedImpl() [d:\build\++ue4\sync\engine\source\runtime\core\private\misc\assertionmacros.cpp:455]
UE4Editor_AudioMixer!Audio::FMixerSourceBuffer::OnEndGenerate() [d:\build\++ue4\sync\engine\source\runtime\audiomixer\private\audiomixersourcebuffer.cpp:507]
UE4Editor_AudioMixer!Audio::FMixerSourceBuffer::`scalar deleting destructor'()
UE4Editor_AudioMixer!TArray<TSharedPtr<Audio::FMixerSourceBuffer,0>,TSizedDefaultAllocator<32> >::RemoveAtSwapImpl() [d:\build\++ue4\sync\engine\source\runtime\core\public\containers\array.h:1565]
UE4Editor_AudioMixer!Audio::FMixerSourceManager::UpdatePendingReleaseData() [d:\build\++ue4\sync\engine\source\runtime\audiomixer\private\audiomixersourcemanager.cpp:2613]
UE4Editor_AudioMixer!Audio::FMixerSourceManager::ComputeNextBlockOfSamples() [d:\build\++ue4\sync\engine\source\runtime\audiomixer\private\audiomixersourcemanager.cpp:2463]
UE4Editor_AudioMixer!Audio::FMixerDevice::OnProcessAudioStream() [d:\build\++ue4\sync\engine\source\runtime\audiomixer\private\audiomixerdevice.cpp:534]
UE4Editor_AudioMixerCore!Audio::FOutputBuffer::MixNextBuffer() [d:\build\++ue4\sync\engine\source\runtime\audiomixercore\private\audiomixer.cpp:186]
UE4Editor_AudioMixerCore!Audio::IAudioMixerPlatformInterface::RunInternal() [d:\build\++ue4\sync\engine\source\runtime\audiomixercore\private\audiomixer.cpp:685]
UE4Editor_AudioMixerCore!Audio::IAudioMixerPlatformInterface::Run() [d:\build\++ue4\sync\engine\source\runtime\audiomixercore\private\audiomixer.cpp:720]
UE4Editor_Core!FRunnableThreadWin::Run() [d:\build\++ue4\sync\engine\source\runtime\core\private\windows\windowsrunnablethread.cpp:96]
Is it really a bug with the new audio mixer or are there any changes required to the project code / settings?