I can confirm this as my brothers Creative Soundblaster Live! 5.1 also has this problem. This is an ALSA problem as alsamixer doesn't mute rear/center either.
** Bug 51530 has been marked a duplicate of this bug -- Master Volume Doesn't effect all channels https://launchpad.net/bugs/59493 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
