You can for instance mod music of the following faction/cultures: northmen, gondor, elves, [mordor, Isengard, orcs], [harad, rhun]. So if you mod music of mordor, you also hear this music when playing isengard. So it's not possible to individually mod music of all the factions. It's not a mod specific thing, but also apply for vanilla version.
As far as I know that is not possible without changing th whole mod.
That's correct. If you want a excellent (

) tutorial on how to add your own music to TATW just go here:
http://www.twcenter.net/forums/showthread.php?t=257219 