Ope - I am guessing it's not your CoreGFX2 but the SCD attachment. a few months ago I sold a duplicate bundle to a fella at Belgium on Ebay, and after he received it he claimed it was having the same problem as you. I tried to troubleshoot the issue with him but ended up giving him 25 Euros + 3 free games since it was too expensive to have him ship it back, but from that experience (and choosing to trust he wasn't scamming me), I think the SCD add-ons has the sound issues due to caps as the Duos. Although I don't have that issue with my other SCD addon unit, I have seem on YahooJP auctions a few loose SCD addons mentioned with the same issues (low sound).
IF the AV port was the culprit then it's a nice simple prob !
Please do keep us posted after you pop it open and if you can confirm the Caps are the root cause... I'm interestd to confirm my theory and if that is the case then I need to pick up a few spare caps for future backup purposes too ...
Yea when I get my gamebit I'll inspect the SCD.