As rusyaas said, everything new is prefixed with "bgm_mj."
Here's the names for all the new battle music.
- bgm_mj_vs01 - Rocket Grunt
- bgm_mj_vs02 - Giovanni
- bgm_mj_vs03 - Archie/Maxie
- bgm_mj_vs04 - Cyrus
- bgm_mj_vs05 - Ghestis
- bgm_mj_vs06 - Lysander
- bgm_mj_vs07 - Ultra Recon Squad Battle
- bgm_mj_vs08 - Ultra Necrozma
- bgm_mj_09 = Dusk Mane/Dawn Wings Necrozma
The mj variants of the Wild Pokemon and Trainer battle themes kept the same name. (norapoke and trainer, respectively.)
The files suffixed with "sysXX" are for the Ultra Wormhole and Mantine Surf.
- bgm_mj_sys01 - Ultra Wormhole Ride
- bgm_mj_sys02 - Mantine Surf
For the new areas, it seems the files listed as "bgm_mj_mXX" are related. - Actually, these are the music for areas within the Ultra Wormhole.
- bgm_mj_m01 - Ultra Megalopolis
- bgm_mj_m02 - I don't have this in my music dump. No idea if it's even a file or not.
- bgm_mj_m03 - Ultra Plant
- bgm_mj_m04 - Ultra Crater
- bgm_mj_m05 - Ultra Desert
- bgm_mj_m06 - Ultra Forest
- bgm_mj_m07 - Ultra Jungle
- bgm_mj_m08 - Ultra Ruin
- bgm_mj_m09 - I think this one is Nihilego's area.
- These next three are the new areas in the regular world.
- bgm_mj_s01 - Alola Photo Club
- bgm_mj_s02 - Big Wave Beach
- bgm_mj_s03 - Battle Agency Music. (atleast the song that was used in the trailer for it.)
For the next section of files, they're prefixed with "bgm_mj_vsdivezoneXX." These are for all the legendary encounters.
- bgm_mj_vsdivezone01 - Mewtowo
- bgm_mj_vsdivezone02 - Bird Trio
- bgm_mj_vsdivezone03 - Latios/Latias
- bgm_mj_vsdivezone04 - Groudon/Koygre/Rayquaza
- bgm_mj_vsdivezone05 - Regi Trio
- bgm_mj_vsdivezone06 - Ho-Oh
- bgm_mj_vsdivezone07 - Lugia
- bgm_mj_vsdivezone08 - Dog Trio
- bgm_mj_vsdivezone09 - Lake Trio
- bgm_mj_vsdivezone10 - Dialgia/Palkia
- bgm_mj_vsdivezone11 - Sinnoh Legendary
- bgm_mj_vsdivezone12 - Giratina
- bgm_mj_vsdivezone13 - Unova Legendary
- bgm_mj_vsdivezone14 - Zekrom/Reshiram
- bgm_mj_vsdivezone15 - Rainbow Rocket Encounter?
- bgm_mj_vsdivezone16 - Xerneas/Yvetal
This is pretty much everything I know about the new music from USUM. I used
Pokeli's Youtube Channel to help identify some of those.