There is a "buffer" transistor that goes between the SID chip (pin 27) and the 8 pin DIN A/V jack. The transistor is a standard NPN type available at Radio Shack (generic 2N2222 or equivalent). It is on the CPU board and is designated as TR2. The original number is a Japanese 2SC458, by the way. Kinda hope that's it. If not... well, you know. :( Another transistor is used as a buffer to feed the internal monitor speaker. It is designated as TR5 and is also a 2SC458. My guess is that if either one of those transistors was shorted, it would kill the audio output to -both- the A/V jack and the internal monitor, since they share a common input. Good luck. 
Ray Carlsen