Ok, it's a nice simple format with an unfortunate wrinkle. There's a table at the front with the start offset of each stream, each stream is standard GC DSP; there's a bit of extra header and padding but it's a standard DSP header inside. I can extract all the DSPs and get dual file stereo pairs. The wrinkle is that the DSP headers are somewhat malformed, the "loop ps" field is incorrect on some files and vgmstream is strict about that. It looks like the loop history is also bogus. They must have faked the header, poorly, rather than use DSPADPCM.exe's. I'd either have to make a new meta reader for this sloppy format that ignores that (would require change to vgmstream, which I'd prefer to avoid), or I could wrap the files in GENH (the GENH meta reader isn't strict at all). Both of which are slightly more than trivial to do, so I'll save this for a later time. Maybe someone will get to the GENHing before me.