BRSTM files go in the /music/strm folder. As for replacing specific songs, each song in Brawl is recognized by a 3-digit code; one letter plus two numbers. Rename your BRSTM file that code and place it in the right folder and you should get a good result.
Here's the track list and code list for every song in Brawl.
I take no credit for compiling this list. I just had one on hand. Hope this helps.
