When you log in a Ubuntu session, a sound is played automatically, which is not comfortable for some users. If you want to stop this sound from playin at Ubuntu 11.04 startup, then follow these instructions:
1. Press the Windows key to bring up the dash and search for "startup applications", then open it.
2. In the list of startup programs, check off GNOME Login Sound, then click Close.
3. From now on, you will never hear a startup sound after you log in.