You can replace any music. When you replace it in the server\regions.cs, you're merely changing how the server handles the mp3. In other words, it just makes it so we, scripters, can more easily choose which music is played by selecting a more descriptive name. When you change them in the config.txt file, you associate that reference with whatever mp3 you chose.
If I were to change tavern04 to void, this is what I'd do.
1. Edit server\region.cs and change tavern04 to void. and recompile the server.
2. Edit music\digital\config.txt and change tavern4 to void
3. Edit data\regions.xml and um....not sure what needs to be done here yet....maybe make the void region and associate the void music to it.
4. Edit dawns music box and change the reference to tavern04 to void. Additionally, you might need to change the cliloc references to tavern04, as Dawns music box uses it.
And, actually, that's about it.
IMPORTANT: Some references are hard coded into the client. DeathTune is hard coded at 42 and cannot change. I mean, you can chose whichever music you want for DeathTune, but when you die, #42 plays. Combat 38, 39, and 40 are hard coded, as is login.