Hi Rob,
I had a GnR with a sound fault and did the following to fix it:
Replace header and connector to sound card.
had a faulty regulator on sound card (2 x regulators to check)
replaced some caps on the sound card which were leaking/bulging

I didn't need to replace the caps this time but there are a couple on the power supply which constantly fail. Most of the smaller caps located near the heatsink on the power supply can leak / bulge.

regards Derek