what firmware is on the infininty ??
i have found in earlyer firm versions of the infinity bugs in the optical out
especially in the 39004
another common failure off distorted sound is DUST in the optical connector
what is the failure of the network adapter ??
possible a fuse is broken near by the Expensian bay connector
also check the connector for damaged pins and yes there it is again DUST
I have repaired LOTS of PS2's only with a can of dust remover
good luck with troubleshooting
MABCOM
A pro installer in the netherlands
www.mabcom.net