A sound sequence, which requires an accompanying minst instrument pack for playback. mseqs can be distinguished from minsts by examining the ninth byte, which is always 04 in a mseq.
Square used variations on the same custom sound tracker for most of its PSX games, identifiable by the use of the file magic tag "AKAO" which begins each sound frame. We don't know much about Cross' tracker in particular, but it seems to be similar to those of Final Fantasy VIII and IX. Unfortunately, the most complete AKAO documentation we have is for the much older Final Fantasy VII: http://wiki.qhimm.com/FF7/PSX/Sound/AKAO_frames . Sections 3.2 and 3.4 of the document archived at http://www.romhacking.net/docs/455/ also contain some information.
If you can read Japanese, http://www41.atwiki.jp/sagafrontier/pages/43.html may also be of interest.
Complete list of games using a possibly-compatible AKAO format:
- Final Fantasy 8 (Tested)
- Final Fantasy 9 (Tested)
- Final Fantasy 2 (presumably the Final Fantasy Origins version, not the NES cart!)
- Saga Frontier 2 (Uses common bank of instruments)
- Chocobo's Dungeon 2
- Chocobo Racing (Sequence data not first?) (Tested)
- Front Misson 3
- Legend of Mana
- Another Mind
AKAO frames from these games (or at least the ones marked "tested") can be played back successfully by Cross' sound engine.
A few maybe-opcodes
- FE 00 - Command terminator. Not to be confused with:
- FE xx 00 - Unknown, but probably relates to a channel number. How many audio channels does the PSX support in hardware - does 24 sound right (the xx I'm seeing here hasn't exceeded 16 [=22 decimal])?
- FD 02 - Unknown. Appears frequently early on as the two bytes immediately preceding FE 00; appears mid-"string" later on.
- FC 02 - Similar to FD 02. Also similarly unknown.
- 8x, 9x - Unknown; seems to be single-byte commands. I've seen 88, 89, 93, 94, 98, and 99. Not all 8x commands are single-byte. Specifically, 85 and 8C seem to be two bytes.