Here's how to do it using AppleScript:
Open Script Editor, and make a new script. If you're an AppleScript newbie, make sure that you type or paste the following into the text box under the Record, Stop, and Run buttons, not in the description box...
Type or paste in the following:
Code:
tell application "Finder" to set volume 0
Choose "Save as Run-Only" from the File menu, and save the file in your Shutdown Items folder, located in the System Folder. Call it "mute volume" or something like that.
Then, make a new script. Enter the following code into that script:
Code:
tell application "Finder" to set volume 2
Again, choose "Save as Run-Only" from the File menu. Place this script in your Startup Items folder. Call it "Restore volume" or something like that.
That should do the trick. Your Mac will now automatically mute the volume when it shuts down, and restore it to a normal level when it starts up the next day.