Pretty simple at all.
All you need to do is to take a music file (instrumental) and change the extension to .ogg
Rename it track1, and put it into the CGames\Toribash [Your Version]\data\music.
There you go!
I am actually listening to a tetris hard trance remix while playing toribash.
A little Hint: don't put too rock/metal songs, they rovine, destroy the game.
And they should be not too long, and something that loops.
EDIT: for example, to understand what i mean with "rovine, destroy", put in the folder a song from Jeff Beck, Scatterbrain, correctly name the file "Track1" and save it as a ogg vorbis.
EDIT2: some people have been asking me on how to change files to .ogg format. That's pretty simple, just search on the internet "Free Mp3 to Ogg converter". If you keep having trouble, well, i seriously don't know what to do. It worked for me by changing manually the extension: Right click on an icon->Rename->Call it track1.ogg