I can confirm the bug with /apps/gnome_settings_daemon/volume_step set to 3 as described by the previous comment
** Changed in: control-center (Ubuntu) Status: Needs Info => Confirmed -- Volume keyboard shortcut fails to increase volume https://launchpad.net/bugs/36302 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs