Audio related crash

Not sure which section to post this in.

I get a crash both in the packaged game and when playing in editor. It’s intermittent, so hard to pin down but seems to be related to sound. I’m not doing anything complex with audio - just using the ‘set base mix’ in blueprints to control volume levels.

Assertion failed: SoundWave && SoundWave->bProcedural [File:D/Build/++UE4/Sync/Engine/Source/Runtime/AudioMixer/Private/AudioMixerSourceBuffer.cpp] [Line: 507]