More modding help can be found here on the official wiki:
https://github.com/Subject9x/battleMETAL/wiki


in your install folder, make the following folder "/metaldata/sound/cdtracks"
(if that folder doesn't work, then try this path: "/user/<user>/Saved Games/battlemetal/metaldata/sound/cdtracks")

only wav, or .ogg works


here is what different tracks should be named:

track002 - main menu
track003 - briefing track

tracks004-008 - actually played during missions

track011 - played during the credits.
track012 - played during cutscenes.

Super easy!