It would be crazy cool to be able to have the mod play our own favorite tracks while sneaking up on a mob, being detected by a hostile mob, or during boss battles

I was wondering if another mod did this, and found a mod called Music Triggers.

If someone is interested in that, here is a possibly relevant page:
https://moddedmc.wiki/en/project/musictriggers/docs/triggers/holder_triggers/mob

I'm going to close this for now, but I can re-open it if someone is still interested and Music Triggers isn't working out for some reason (post reply here if this is the case).

Aye, someone actually suggested hooks into a music mod for this kind of thing, a long time ago...I never got around to it, but yes it's an interesting idea.

I'll leave this open.