Here is all the research I did on the topic.
https://forums.rpgmakerweb.com/index.php?threads/bit-of-testing-with-opus.158209...
How can I attach files here?
Viewing post in OPUS support?
Here is a converted audio file: https://www.mediafire.com/file/kau7l0fs9qtlwah/AUD_BGM_MiasmaLord_Normal.ogg/fil...
Okay, I attempted to convert the Jay's Journey tracks "Starboard Authority" and "Starboard Authority (Alt)" into OPUS files. They play just fine so long as I don't mess with the loopstart and looplength attributes (which, true to what you said in that topic, do not work properly). If I do alter those attributes, then upon playback I'm greeted with:
Failed to set the 'loopstart' property on 'AudioBufferSourceNode': the provided double value is non-finite.
So either there's a previously-undetected bug in my plugin, or there's an issue with OPUS files.
(Actually, I did discover an undetected bug in my plugin. I'll, uh, just go and release a new version for that. But it's not one related to looping or OPUS files.)
Figured it out. The calculated sample rate of the OPUS files I'm testing with is ZERO. There's some math in my plugin that divides by the sample rate at one point, and division by zero = bad result.
If there's some equivalent of the sample rate in the metadata, or if I could fake it somehow, this could be patched.
All this said: The variable mix stuff - the meatiest part of the plugin, and the whole reason I made the plugin in the first place - still work as intended. I haven't yet tested every feature, but the loops seem like the biggest thing likely to break that did, in fact, break.
EDIT: Nope, one more thing is broken: the "Play from" (or "Starting point") attribute hangs the game.