Dunno about the codes, but as for the how to I'm guessing its normal procedure.
1. Rename the brstm to match the song code
2. Put the file in pf/music/strm
3. Run the game
Although the code might require different setup.
Thing is, STGCUSTOM loads the songs from Battlefield by default, I wanted them to load specific songs in the library depending to which STGCUSTOM I'm linking to. That requires one code per STGCUSTOM, which means I need to have 35 codes, and link them to 35 songs in Subspace Emissary...