1. You're going deaf!
2. Headphones Level not high enough?
Everything computerinternal maxed. Balance headphone output maxed. I'm using a pair of Beyerdynamic DT-770 pro, so no headphone volume control.
2.5 Headphones have their own volume level and it is not set appropriately?
3. Bad (broken) cans?
Headphones work like a charm, deliver full, crystal clear sound via the chea built in iMac soundcard. 4 times the level I might add.
4. System sound output level too low?
5. A Leprachaun
Maybe, but highly unlikely.