Can UI mods control which music is played?
 

There was a UI mod that did this back in WoW, allowed the whole game to be re-scored with other in-game songs or your own MP3s... I'd love to have something like this for LOTRO so I can fill it with the music from the movies. :D Does anything like that exist for this game or is it possible?

simple answer, no.

your only able to change the looks of the ui.

and i rather like it like that :)

It's like a Winamp skin or a WMP skin: you just change the way it looks :)