[jtrutan]

Here are the common reasons for the symptom:

1. The jack is improperly soldered on to the PCB
2. The jack and plug are not mating properly. Either the contacts are oxidized, or the contacts are deformed.

What it is probably not:
Chipset malfunction
Defective discrete components (like capacitors, transistors...)

Here's what I do:
• Clean your earphone's plug with a paper towel and some contact cleaner. (I use WD40 or metal polish.)

• Turn-down the volume of your player. Plug in the plug into the jack all the way through. Wear the earpieces. Turn the volume up and see if the symptom is still there.

• If it is, pull the plug until you only hear one channel (either the left or the right). You are trying to determine if the problem is with both channels or just one.

• Plug it in all the way through again and gently jiggle the plug in the socket. It should barely move. If it moves too much, one of the jack's pins is probably loose. The worse case scenario is the copper film to which the pin is soldered to has been delaminated from the board.

At this point I'd open it up and try to fix the improper solder. Or if there is any copper film delamination, I'd use magnet wire to 'bridge' any gaps that may have formed.

Hope this helps.