I remember changing it to the puddi puddi song. It is possible, unless something was changed.
edit: Just tested it again, it works. These are the steps I performed:
Find whatever music file you want. Use a file converter to convert it to mp2. I use
FormatFactory. It's free and awesome.
Rename it to 'rugged.mp2'
Drag it to Legions Overdrive\
live\legions\data\sound\music
Depends on what version your launching, either Live or Public Test. I doubt anyone will be using Public Test anymore, but I felt the need to specify anyways.
It'll ask you if you want to replace the existing 'rugged.mp2', click yes on that bitch.
Set it to Read Only.
Start Legions and it should work.