I am trying to do this myself, and can't do it. Maybe it is not possible to do in a mod?
Correction: I have found that the creator of the Crete campaign, LouisM, has managed to get the music file to change for his campaign. I seem to be doing the same as what he has done, but it does not work for my mods, I will have to investigate more!
Correction2: The difference between the Crete campaign and what I am trying to do is that the Crete campaign is accessing a different music file that is already in the main game installation, while my mod has it's own "music" files. So maybe the game does not read music files from a custom mod/scenario folder?
Correction3!: I have found another scenario, JCB989_FUEL_DEPOT_3, that uses new music for the Deployment phase. So that at least is possible. I have not found any user created scenario that has new Battle music or sounds, which is what I am trying to do.