Crash in FMOD5_Thread_SetAttributes

if I’m not mistaken, there’s no way to have an internal quad channel surround mix get downmixed to stereo in FMOD Ex, right?

I think it should be fine to ignore that error, are you just calling FMOD_Sound_Release on each of the subsounds to free them?
If you call System::setSpeakerMode(FMOD_SPEAKERMODE_STEREO) that should automatically downmix everything to Stereo.

I meant as in work in surround which gets up/downmixed to the actual hardware speaker setup like in FMOD 5, rather than having numerous edge cases for various speaker layouts

oh and another thing to note, the crashing is seemingly related to channels as after reimplementing them, it’s back

Oh I see, you are correct there was no automatic downmixing in FMOD Ex.

What part of channels have you reimplemented? Is it the calls to Channel::setPosition or Channel::setMixMatrix as in your previous code snippet?