Not sure if this will do you any good but could you swap the left with the right speaker and see what it does?

flipping the speakers could narrow it down to whether its the actual speakers or the wiring.

I had the same problem on my TAF. The only speaker working was top left backbox speaker. My machine had been in a container for years from Germany, so who knows what goes on inside these machines. I checked the other two speakers with a 9v battery and 2 wires coming off the + and the negative. Touched them to the leads of the speakers, and after a couple of times, they produced static. Now they work....go figure? Maybe they were stuck.
All i know is that my pin sounds a lot better. Worth a try, at least you will know if speakers are fried. ie: you hear no static. Another question answered was which speaker lead was positive and which negative since they are not marked. If the speaker pushes out, you have it connected correctly. If it sucks in, you have the positive of the battery hooked to negative lead of speaker.