I think it is Xorg related; this bug has been already reported on their bug website.
Check https://bugs.freedesktop.org/show_bug.cgi?id=7625 -- no multimedia keys when using evdev with mouse https://launchpad.net/bugs/44169 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
