Some users are finding that after rebooting their Mac, the previously changed sound levels have reverted to a default setting. For some users the sound level is set at a minimum, while others report that the setting defaults back to fifty percent.
Apple Support Discussions user "Mccabbe" reports:
Hi, i upgraded my computers this week end (iMac, macbook, mini) from Tiger to Snow Leopard. All went great except one little bug : the sound volume resets at each reboot, for example on the iMac it is always at minimum after each reboot. This occurs on all 3 computers.
Another ASD user echoes the issue:
This Snow Leopard sound issue sure gets around. Mine stared some time after the initial SL install. I have since done a compete erase and reinstall with no charges to my sound issue which is;
I can set my system volume level to about 10% but when I restart or startup the iMac the volume level defaults to the 50% setting. I have tried all the suggestions within this post with no fix. Applecare has no idea.
The issue seems to lie in a corruption of the audio preference files in Snow Leopard. To alleviate this problem, navigate to the preference files and move them to the Trash.
Macintosh HD > Library > Preferences > Audio
The files will be: