I do not like the startup, shutdown and notification sounds that desktop operating systems make. To have a completely silent experience with Ubuntu, do the following:
Turn off all sound alerts
Go to System → Preferences → Sound. Choose the No sounds sound theme.
Turn off the login sound
This is generated by GNOME when you login. Go to System → Preferences → Startup Applications. Disable GNOME Login Sound in the list.
Turn off the startup sound
Ubuntu makes a startup sound (typically an African drumbeat) when it presents the login screen. Go to System → Administration → Login Screen. Disable the Play login sound option. The system will request for superuser privilege to apply this.
Tried with: Ubuntu 10.04 (Lucid Lynx)