In Ubuntu 10.10, Maverick Meerkat, I am unable to find a way to change the startup or login sounds. Therefore, I have to find the exact location of the sound files.
In /usr/share/sounds/ubuntu/stereo, one can replace desktop-login.ogg and system-ready.ogg. Replacing these two sound files with your own will play your sound file.
Desktop-login.ogg – The sound that is played when a user successfully logs in.
System-ready.ogg – The sound that is played when the Ubuntu login screen is displayed.
In order to create ogg files, Audacity can be used to convert non-ogg sound files to ogg format.
To be able to copy and paste files in the file system, http://aprivatebeach.com/blog/2008/10/create-new-menu-item/ will help.