It's easy to disable the Windows login sound and related sounds. That's done by going to Start --> Settings --> Control Panels then selecting the Control Panel "Sounds and Audio Devices".
Click on the "Sounds" tab and it'll look like this:
Scroll down just a bit and you'll find Start Windows and Windows Logon:
Those are the two you want to change or disable entirely. To change them, click on the pop-up menu that currently says "Windows XP Startup" on my screenshot above.
To disable them is a bit unintuitive: You need to also click on the pop-up menu, but this time you're interested in the top option:
Select (None), click on "Apply" and you should be able to restart your computer with considerably less interruption next time you're in a meeting or at a class.
Source:askdavetaylor.com