Comment 71 for bug 788540

This is a known issue with Lenovo Thinkpads. The workaround is to use a shell script and a recompiled kernel module. You can find the steps here:

http://askubuntu.com/questions/125367/enabling-mic-mute-button-and-light-on-lenovo-thinkpads

It would be nice if this could work out of the box, though.