Good timing! I was about to give a bit of an update, I now have it working %95 well, but not perfect. While I can now understand the audio coming though, it is rough at points and trying to smooth that out.
The sample itself is set up as “item->samples = (float *) malloc(sizeof (float) * frames * channels);” when first created, so just taking “sizeof (float) * frames * channels” for the FMOD_CREATESOUNDEXINFO length has solved most problems.
Here is a pastebin of the rough code I have made on both the side that pulls from TheoraPlay, and the side that uses FMOD, culled of misc error checking and such of course. https://pastebin.com/pGc6nhix
In case it helps here is the TheoraPlay’s authors own example how to use it with SDL audio http://hg.icculus.org/icculus/theoraplay/file/tip/test/simplesdl.c