So I got a Model 1 Sega CD recently and I was able to fix it up so it loads, reads, and ejects properly. The glaring problem now is that it has no audio. The only audio the Sega CD produces is a constant thumping buzzing sound that is unaltered by any CD reading.

What I know so far:
1) It's not the Genesis' audio, that's fine.
2) I've tested it with 2 different model 1 Sega Genesis'. Same thing.
3) I heard the same sound from the headphone jack, so it's not the RCA outputs.
4) It can read a game OK, so it's not the laser. (Right?)
5) I've cleaned the ribbons connecting to each board, but I haven't messed with the ribbon underneath the laser.
6) I've cleaned the contacts (Sega CD and Sega Genesis) that both plug into the side "bridge".

Suggestions? Thanks,