It seems that I cannot adjust the system volume when I am not logged in in Linux (using Ubuntu). Sometimes I come to a library and I want to turn on my laptop, but it is very embarrassing when it generates very loud sound during startup. I still want to have a login sound however in non-quiet areas. The problem can be easily solved if I can adjust the volume before login, but how can I do that?
Your laptop should have volume adjustment button(s) on (or above) your keyboard. This doesn't control the Sound in the Operating System, but controls the amount of sound put out through the computer's speakers. Once the login is completed (or your out of a quiet area) you can re-enable the volume again.
Now, if you don't have one - in which you should, you may be able to fix it by holding the speaker "hole" on your laptop.
The first idea would work perfectly.