How do you fix earphones that one side is silent?

Your best chances of repair are if you contact Sony and ask them.

If This problem is only happening when you use the headphones on one device such as an iPod, the circuit in the device may be damaged. I won't get too involved into how to fix this, though.

But if Sony won't help, your warranty is out on them, your device you are using if not the problem, and you need them badly, I may have a fix:

Open up the earphone that is malfunctioning

check the solder connections that connect the wire to the speaker

if they connections are loose, you have two options:

buy new headphones

OR

If you can solder, Connect and solder the correct leads to the corresponding places.

If the solder connections are fine, check and see if the wire is frayed or damaged anywhere.

If it is, you have two options:

buy new headphones

OR

salvage an old headphone's speaker wire from somewhere else and try to remove the old wire and use the salvaged one to replace it. (this will require that you are quite good at soldering.).

If only a small part is damaged, try and use a smallpiece of the salvaged wire and splice it into the old wire. If the wire is very small, I recommend that you just buy a new pair of headphones.

if you can find out if the speaker is blown out or has ripped itself, you can try this trick I learned in MAKE magazine:

Get a bottle of rubber cement

very very very carefully apply a few thin layers of rubber cement to the ripped or torn area. You might wait for the glue to dry and then reapply. Make sure that if you do do this, you do not over do the rubber cement. Otherwise, the speaker will just sit there and be "frozen".

If you have access to an oscilloscope and know how to use one, you might try to probe around in the circuit and see if you can find interference anywhere that is causing the problem.