1) Delete com.apple.systemsound.plist (in /Users/youruser/Library/Preferences) and restart. If that doesn't work, move to option two...
2) Restart your computer and zap the PRAM (Command-Option-P-R) and wait for three chimes (three startups), then let go. If that doesn't work, try option three...
3) Boot up from your system CD/DVD and see if you can get any sound out of the computer by clicking around where you aren't supposed to (to get that sosumi/beep tone). If no response, try option four...
4) Backup your computer with SuperDuper or Carbon Copy Cloner and reformat the hard drive and reinstall the OS. If not, try option five...
5) Purchase a new Mac. But seriously, I'd probably take it apart and check all the cables inside. Then I'd purchase a new Mac.