I thought I was losing my mind or my hearing. I would put my headphones on and I could tell the music was just a bit off. Then I read this: http://support.apple.com/kb/TA22305

In some cases the audio balance may unexpectedly drift towards the left or right channel. This can happen if you rapidly press the volume up or down keys while the computer’s microprocessor is under heavy load.

Their Fix?

Use the Sound preference pane to restore audio balance.

  • Choose Apple Menu > System Preferences.
  • Choose View > Sound.
  • Drag the audio balance slider to the desired position.