No Sound through headphones on an iMac

[Hugh Caley]

Fedora 17 on a newer iMac, the sound is pretty much inaudible with 
headphones plugged in, but the speakers seem to work fine with nothing 
plugged into the headphone jack.

I've check alsamixer and the Gnome3 Sound control panel, nothing seems 
to be shut down when the headphones are plugged in.  No errors are 
indicated in the logs.  Headphone jack works fine under OSX. Anyone have 
a thought as to how to work around this?

Hugh