You can change startup (or any other) sound in Ubuntu as follows:
What you will need: startup sound in ogg format. You can find some ogg sound here
Browse to following paths
Look for desktop-login.ogg in the list. If you don’t want to hear anything at startup, delete this file.
If you want to change the sound, replace this file with your favorite track and with the same default name desktop-login. Save and close
You will need root permission to copy the file into this directory which can be done by running nautilus as root. Nautilus is a graphical application and, as such, it is better to open it with
gksu instead of