I believe that this is not yet possible under OS X. The System Preferences is one area that is not yet scriptable. There is a "set volume" command, I tried it and it does adjust the volume, but not the main system volume, it only seems to affect the volume of any sound used in the script. Play around with this script, changing the number from 0 to 7:
tell application "Finder"
set volume 7
beep
end tell
BTW, I found this in the Standard Additions dictionary.
Steve W