Each time when you login to Ubuntu, it plays a login sound. If you don't like to listen to it each time you login, you can easily disable it, or you can even change it to your favorite sound.
To disable the login sound:
Go to System > Preferences > Startup Applications.
Under the "Startup Programs" tab, untick "GNOME Login Sound"
Click Close
To change the login sound, tick the above "GNOME Login Sound" if it's unticked, then follow these steps:
Press Alt+F2 to bring up "Run Application" window.
Paste gksu nautilus /usr/share/sounds/ubuntu/stereo into the box, click "Run" to open Nautilus in the right folder.
Rename the original file desktop-login.ogg to another such as desktop-login-original.ogg for backup
Copy your sound file in ogg format to the above folder and name the file as desktop-login.ogg
Log out and log back in to listen to the new login sound.