well, the aboveseq files are the land sounds the belowseq are the underwater sounds and the geoseq line up with the geoscape sounds, although i'm sure none of this is news to you. it also seems like there are a lot of instrument sounds, which i suspect were arranged and played in code to produce certain musics.
basically you'll want to make an extrasounds ruleset that inserts the aboveseqs into battle.cat, the belowseqs into battle2.cat and the rest into geo.cat. as you've pointed out, the orders in geo.cat are plain as day (the hard part is identifying them)
as for the battle cat files, our structure is:
0-13 are movement sounds
14-47 are the weapon sounds
48-66 are the various unit death sounds
67 is the ambient sound effect
don't worry too much about the soundDefs thing, that's just how i split TFTD's single cat file into multiple, usable ones, instead of rewriting a mountain of already working code. basically i adjusted the data to fit the code instead of the other way around, because it made a lot more sense.
be forewarned however, that the extrasounds will be indexed to avoid conflict, and they'll be indexed according to xcom1, not xcom2 because full modding support for tftd is not yet implemented. this is all tech talk, but what it means to you is: some of the higher numbers won't get replaced properly until full support is added.
