By default ubuntu 12.04 startup sound was disabled.If you want to enable startup sound in your system follow one of the solutions
Click Startup Applications in the system menu at the top right of your computer and make sure the GNOME Login Sound box is checked
You need to add the following startup item to the startup applications
Name: GNOME Login sound
Command: /usr/bin/canberra-gtk-play --id="desktop-login" --description="GNOME Login sound"
Comment: Startup Sound