Unfortunately, the PS3 doesn't output audio through its USB ports. It only outputs voice chat audio.
The only way to do that is to either hack your USB end of the headphones as jadberro said (although I would recommend hacking a cheap headphone first to try it out) or use this item.
When and if you can find such an item, plug your headphones into the USB end. The RCA female should go with the RCA male from the PS3. You then go to the audio settings and and select RCA output and it should work fine.