Comment 3 for bug 1318233

Yes it seems to be the expected behavior by light locker.
They have added an option that only lock once the screensaver is deactivated, that should mostly fix your problem (see this for details : https://bugs.launchpad.net/ubuntu/+source/light-locker/+bug/1296456)
It seems the only thing that can be done on gmusicbrowser's side is to add an option to deactivate the screensaver while music is playing (in addition to the existing option that disable the screensaver while it is playing in fullscreen mode)