All three of the Gorfs I have had ended up having some kind of audio problems. I agree with Edward, definitely look at the amp. If you want to verify if its the TIP31 swap them side to side and see if the problem moves.
All three of mine needed a TIP31 on one of the channels.
2 had blown speakers. The voice coil was seized solid in both cases. The channel with the blown speaker was the same as the one needing a TIP31 in those cases.
The AMP board also has a 2N4403 in each channel. I also ended up replacing it on the last Gorf I had. In this case I replaced the blown speaker and the associated TIP31 and had sound. While letting it burn in it would randomly crash and lock up after a hour or so. I could disconnect the AMP and it would run forever. I also want to say that the channel I had fixed always had a constant tone coming out over it during the crash/lock up. I swapped in a new 2N4403 on a lark and never had the problem again. My assumption was it was failing, overdrawing current from the supply at some point (drive to ground???), and crashing the board because the voltage dropped. Total guess. I did not go back and dig into it to understand the detail of the circuit once it was running.
Some notes I made to help me isolate a given channel by sound.
The volumes knobs. The left is channel 2, right knob is 1.
Channel two, among other things, holds the background "dum dum" sound on Astro Battles as well as the enemy laser sound on Laser Battle.
Channel 1 is the Gorfian voice (when the game talks) and has the fire sound of your ship.
I do not recall which speaker is which. Once you figure out which channel you have working you can derive the speaker for it.