Have had the same setup/issue you have since buying mine about 2 years ago, despite going through 10.6 & 10.7. There is no fix as far as i know. Annoying right? Also don't forget to go to Audio Midi Setup and configure your speakers as 5.1, definitely sounds better with 5.1 content.
I'm kinda disappointed with the sound as of late. My $100 head phones sound better; looking around for ways to improve it, just doesnt sound quite as crisp or well rounded. Tips?