you know how the powerboard is connected to the GC?
Well, ER once told me that when his sound stopped working, it was because the powerboard wasn't regulating the 12v line anymore. I assume your input is around 12v, so just extend a wire from your main 12v line to the 12v in contact on the actual GC.
I hate to say this but there is probably nothing more you can do. Sound is common problem that GCP portablizers deal with. Sound is the most sensitive circuitry in the Gamecube IMO.
If SS or someone else knows of a solution besides the rewiring you tried then great! But I do not sadly :/