The problem is the async loading. When the NiagaraDataInterfaceAudioSpectrum loads async, it checks to see if it’s on the main game thread when RegisterToSubmix.
One fix is to disable Async Loading Thread in project settings
OR
Go into Engine\Source\Runtime\Engine\Private\AudioThread.cpp and comment out all
check(IsInGameThread());
Since the audio thread runs in it’s own thread and the source file is called AudioThread.cpp not sure why there is a check to make sure it’s on the main game thread. This check is ignored by the editor.
I’ve tested both. And since I want async loading, I went with option 2…
//check(IsInGameThread());
This is still happening in 5.0.3 (github 5.0 branch latest)… the Bug is flagged not to look at until 5.1