Hi @diregoldfish,
A couple of questions:
What version of UE4 are you using?
Are you using the New Audio Engine?
Are any of your music SoundWaves set as Streaming?
Have you watched my video on creating a complicated interactive music stitching system?
In your Project Settings, go to Audio > Quality > Quality Levels > 0 > Max Channels, what is it set at?
What about Max Channels on your specified Platform? (Windows or whatever) What is it set at?