(Not necessarily an FMOD solution, but please read the thread for context)
Last update: I DID have a misunderstanding with D++, or more specifically with Opus.
Opus expects 16-bit signed integer PCM data. But because it’s a C library, it wants that data cast to unsigned char, which is 8-bits each. I assumed Opus wanted 11520 samples per frame, but instead, it wanted 11520 bytes, and thus only half of what I expected would be sent with each Opus frame, before I deleted it from the PCM buffer.
Thank you sincerely for the help, and hopefully this thread can help somebody in the future!