are you using the correct cable plugged into the
correct place?
this is usually the biggest cause.
sound was mono, and you need the proper wiring, and your tv/etc may
only accept
mono sound on a specific input
One thing I've been wondering about is if this could be caused by
problems with the 12v supply feeding the SID. Is there anything else
in the C64 that uses that voltage or is it just for the SID?