Hello,
I found this bug on Launchpad https://bugs.launchpad.net/ubuntu/+source/gmusicbrowser/+bug/784115 (https://bugs.launchpad.net/ubuntu/+source/gmusicbrowser/+bug/784115)
On Ubuntu 12.04, gmb does not appear in Unity settings -> Default applications -> Music.
Nor does it appear in the list of recommended applications in Nautilus properties for mp3 or ogg files.