I had this little problem happening over the last couple of weeks (as I began to notice it), I initially thought it was due to me installing Snow Leopard, however with a bit of research its seems the problem is far more widespread. In a perfect world you Mac should, when you restart, have the same volume as when you log off and do the restart. This is not the case, I personally have a muted volume when I log on, although it could take any value in between. The solution to this problem is simple however you can easily overlook the problem.
There are two solutions to this problem depending on your Mac’s current state. The first solution is to check for any applications or System Preferences you have installed. For example a year and a half ago I mentioned about Disabling the Boot Up Sound, I installed this app and was happy with the results. Over that period of time I completely forgot it existed. As a result I couldn’t figure out why the boot up volume and subsequent log on sound was already zero. Therefore:
Solution Number 1: Check any applications and preference and adjust your settings.
That should work for most people, however if you don’t have anything installed that wont work. The second result is to delete some preference files. To get around this problem you have to delete the right preference files. Navigate to the following folder, Macintosh HD > Library > Preferences > Audio and delete com.apple.audio.DeviceSettings.plist and com.apple.audio.SystemSettings.plist. New preference files should be generated when your Mac runs the required pieces of code. This should solve the problem.
Solution Number 2: Delete the correct preference files.
This should, in theory, solve the volume resets and keep your Mac’s volume at the same volume as when you log off and shut down. If this doesn’t it could be a more sinister problem.