When you set the volume of your system to a certain level, this setting should be preserved when you reboot your system or log out and back in. Despite this, some people have reported a problem where the computer will not save these settings, and will either revert back to another volume level or be set to mute.
Apple discussion member "AndyNylesWillRiot" writes:
"I just noticed since i've had SL, no matter how loud i leave the volume when i shut it down, every time i boot up he volume is muted. I just know this didn't happen with Leopard so i was just wondering if there is any solution to this? or if it's just some sort of new feature?"
If this happens it is definitely a bug or problem with the configuration files that the system is using to set up the audio. The system primarily uses two preference files for system audio settings, and also stores some audio settings in the system's PRAM. While this has happened for some after upgrading to Snow Leopard, the same configuration files are used by Leopard and it is possible for Leopard to be affected by this problem as well. The easiest solution for this problem is to remove these configuration files and allow the system to remake them, as well as reset the computer's PRAM.
Remove Configuration Files
The two configuration files are located in the /Macintosh HD/Library/Preferences/Audio/ folder, and are called "com.apple.audio.SystemSettings.plist" and "com.apple.audio.DeviceSettings.plist." Go to this folder and remove these files, and then restart your system and the audio settings should be functioning normally again. These preference files hold settings such as bit rate, the number of channels the system is set up to use, and sound volumes, etc., so you may need to set these up again.
The PRAM holds a few audio settings which at times may be used in place of those that are in system configuration files. The PRAM settings can be reset by rebooting the system and immediately holding down the options-command-P-R keys at once. The computer will keep resetting and making the boot chimes as long as these keys are held. Let it cycle a couple of times and then release the keys and allow the system to boot normally.
NOTE: If you have a firmware password set, you will not be able to reset the PRAM until you disable the password.