Very well, then. I do find myself wondering what multiplayer missions you've been playing though, since, believe it or not, music in multiplayer works just fine.
Which is to say, there *are* multiple tracks played at different times when you're in a dogfight, new enemies are arriving, you complete an objective, and more. In addition to that, the generic action music is selected from 3 different tracks so you're unlikely to keep hearing the same one constantly throughout the mission. Further, mission designers have the option to change the current soundtrack or even play music and sounds directly from a file (although this does have some problems with multi - but that's a bug and will be fixed eventually).
The only way music in multi is "broken" is if either the mission designer forgot to add the music, or the mod being played has screwed up in such a way that the music system can't work properly due to table and/or media problems.