I'm not 100% sure but you'll probably have to put the line "scan_module_music = 1" (without quotations) in the module_info.txt file, just like you'd put "scan_module_sounds = 1" if you wanted to replace some sounds. Of course, that might not work at all.
Edit: Looking through the folders, there's a "music.txt" file in the module directory (Modules/Native, for instance) with the names and extensions of music files that can be found in the "Music" folder in the main Mount and Blade directory. Then there's a bunch of numbers. Honestly I don't know what you should do, but it probably involves the music folder and that text file.
Edit2: http://forums.taleworlds.com/index.php/topic,53376.0.html