After restarting in Mac OS X 10.4.5 or later on some Intel-based Macs, the output volume level and balance settings for headphones or other devices connected to your computer's headphone jack may revert to the default settings. The default settings are: Mute is disabled, volume is set to 56% of maximum, and balance is equal for the left and right channels.
Adjust the volume level and balance as desired after restarting the computer.
This document will be updated as more information becomes available.