With a vanilla kernel 2.4.30 one can get keycodes from the following keys:
Fn + F3: 223
Fn + F5: 174
Fn + F6: 176
Fn + F7: 160

Install hotkeys (apt-get install hotkeys or download it from

Copy the definition-file flybook.def to /usr/share/hotkeys/.

Add to/Modify /etc/hotkeys.conf:

If you now run hotkeys you can control the volume with Fn+F5 and Fn+F6 and mute the mixer with Fn+F7.

Add "hotkeys &" to ~/.xinitrc resp. ~/.xsessionrc so it is run each time you start the X-Server.