If you want to remove the startup or login sound of Ubuntu, then here is a simple solution.
All you have to do is to remove or disable (uncheck) the GNOME Login Sound from the Startup Applications.
To know how to Add and Remove the Startup Applications in Ubuntu, check out the following post:
Startup Applications Ubuntu / Linux
Please note that the provided solution is specifically for Ubuntu 10, but could be used on the other versions as well.