Let's make sure something is actually "broken" before you try doing any repairs...
Reference page #7 of the manual (available from IPDB if you don't have one - see:
http://www.ipdb.org/machine.cgi?id=2165)
Note that you can change the sound settings between "tune", "chime", and "noise" depending upon how you configure DIP switches #8 and #32 under "Sound Option" at the top of that page...
You may have a flaky DIP switch, or if you were mucking around in the head recently, may have accidentally changed one. Regardless, try these before assuming you have a bad sound board, assuming the sounds are just "different", vs. missing/corrupted.