Sorry for the obvious question but do you have the headphones plugged into the headphones jack or the output on the breakout cable?
Also, do your headphones have an adapter on it? Mine does to go from 1/8" to 1/4". The plug should have 2 rings on it to indicate stereo; if it only has one its mono and that could be it.
Did you troubleshoot your headphones on another system to make sure both sides work?
Thats all I can think of.