And speaking of the headphone jack, I had somthing very similar happen on my keyboard (GEM WK8), all of a sudden, no sound out of the right side. Turned out to be the headphone jack, it didn't re-connect the speaker when I pulled the headphone plug out (sticky or dirty contact points?). Both channels worked with the headphones plugged in, both channels worked when connected to an external PA. I pulled out the plug on the headphones and got some cracklin' out of the right side speaker. I inserted and pulled out again and the right side came back.

I haven't plugged in the headphones since....

-mike