The only way for music, is to rip them to .wav format, and include them as ambient sounds placed in the area. Then, use SoundObjectSetVolume to define its level of volume.
// Set the volume of oSound.
// - oSound
// - nVolume: 0-127
void SoundObjectSetVolume(object oSound, int nVolume)
This way will require a .hak download or an ambient download to dump to /ambient.
Either way, it's a download. There are no in-game script commands to adjust music level.
FP!