If you want to change the music entirely, you could simply replace the corresponding .mp3 file in your client folder's BGM directory. Doing this would change the music on every single map that uses the new file. For example, replacing 87.mp3 client-side would change any map's background music that has the 87.mp3 defined in data/mp3nametable.txt.
If you wanted to add a new .mp3 file and change a specific map, you would change the map's file definition in data/mp3nametable.txt.
gef_fild02.rsw#bgm\\35.mp3#
gef_fild03.rsw#bgm\\35.mp3#
gef_fild10.rsw#bgm\\35.mp3#
in_orcs01.rsw#bgm\\35.mp3#
Here, we can see that multiple maps use 35.mp3. Instead of changing the song for all four maps that use it, we could change a specific map by adding a new file to the BGM folder and defining it. For example, if I added a new .mp3 and called it 1001.mp3, I could then change:
in_orcs01.rsw#bgm\\35.mp3#
to:
in_orcs01.rsw#bgm\\1001.mp3#
This will change the background music on in_orcs01 instead of changing it for the other field maps that use 35.mp3. This is particularly useful when adding custom maps or just getting a different feel for an existing one.