Lenovo x61s volume keys not working (regression)

In 8.04, the "Volume Up", "Volume Down", and "Mute" special multimedia keys on the Lenovo X61s laptop worked out of the box. However, these keys no longer work in 8.10.

Previous functionality: pressing "volume up", "volume down", or "mute" keys raised an informational overlay dialog indicating the volume and changing the volume accordingly.

Current functionality: pressing the keys does nothing out of the box. After going to "System->Preferences->Keyboard Shortcuts", I can reassign the "volume up" and "volume down" keys to the appropriate values. However, the Keyboard Shortcuts dialog does not recognize presses of the mute key, and will not accept assignment for it.

Running Ubuntu 8.10:
2.6.27-7-generic #1 SMP Tue Nov 4 19:33:20 UTC 2008 i686 GNU/Linux

Lenovo X61s laptop.