How to enable the ‘Mute’ button on a X200?

To enable the ‘Mute’ button on a Lenovo X200
(and probably some other notebooks as well)
it’s required to set an ACPI mode switch for the kernel.

Just open menu.lst in an editor

root@x200:~# nano /boot/grub/menu.lst

And add this switch: acpi_osi=”Linux”

kernel		/boot/vmlinuz-2.6.31-14-generic root=UUID=kernel /boot/vmlinuz-2.6.31-14-generic root=UUID= … ro quiet splash acpi_osi="Linux"

After a reboot the ‘Mute’ button will be recognized instantly.