It would be similar to switching the voices from the English pack to Korean or Japanese.
Basically you have a folder structure in <Aion>\L10N\ENU that mimics the <Aion>\Sounds so you have things like <Aion>\L10N\ENU\Sounds\voice\attack that contain the sound files (in Ogg Vorbis format). Obviously you would be replacing things in the "music" subdirectory.
I'd suggest you start searching with how to extract the Aion soundtrack.
Note: I haven't tried this with the music, but theoretically it should work. Otherwise you'd have to repack the .pak files in the Sounds dir.
"Oh, hell no," Kylar said. "You cannot fly. Tell me you can't fly."