Looking at the back, the audio circuit is what extends the furthest to the right. If I wire my own audio amp, can I cut this part off, something like this?
If you use your own audio chip then you can cut it off. Altough you have to wire the composite video line directly. Check my other pics for the pinout.
Your video decoder chip is probably not getting 5v as you cut through the contacts near the regulatir. Check with a multimeter that you have. 5v connection if not rewire it.