Well, I am certainly no expert but the first thing I would look for is any solder that may have dropped on the N64 by accident. Especially near the pins that carry the audio on the multi av port. Here is website you can look at that has the pin-out info. http://pinouts.ru/Game/n64video_pinout.shtml Assuming you followed all the way through with the mod (connected RGB lines, cut the trace to pin 9, and connected 7 to 9 inline resistor), you could have done something to pin 11 or 12, both of which handle audio. Did you accidentally connect pin 9 to one of those? Maybe one of your wires is not well shielded and is touching one of those pins? Can you give any more details?