I couldn't find this covered, and it's fairly simple, but I didn't see any references to it elsewhere, either. I recently bought a Kensington keyboard to replace my broken Apple keyboard -- I figured it was worth saving a few dollars. Then I realized that it would be awfully hard to live without my volume adjustment keys! Enter AppleScript and Xkeys.
I simply mapped AppleScripts that adjust the volume (up, down, and mute; courtesy of a Google search) to function keys F13 - F15. Voila! Works exactly like my other keyboard!
[robg adds: Though the author offered to share his scripts, I figured I'd just list three that worked; these may differ from those of the author, but I tested them and they work...
Volume Up:
set volume output volume ((output volume of (get volume settings)) + 3)
Volume Down:
set volume output volume ((output volume of (get volume settings)) - 3)
In both of the above, feel free to change the 3 to another value; go outside +3 or -3, and the volume change per key press increases; go the other way to reduce the step size.
if output muted of (get volume settings) then
set volume without output muted
else
set volume with output muted
end if
Though they were tested, I didn't actually use Xkeys to assign them, but I wouldn't expect any issues...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051107024046848