If you are one of the early users who have already upgraded to Ubuntu 11.10 Oneiric, you will discover that the startup login sound is activated by default. Worst still, there is no way that you can turn it off.
The reason for this is because the login sound entry (in the Startup Applications) is hidden by default. Why is it hidden is unknown, but here’s how you can unhide (and disable) it.
Note: This is an annoyance found in Ubuntu Oneiric beta. It might (or might not) be fixed in the final release.
1. Open a terminal and type in the command:
2. At the end of the file, change the “NoDisplay” from “true” to “false” (without the quote). Save and close the file.
3. Now, go to the Startup Application (from the power icon at the top right corner) and you should see the Gnome Login Sound entry. Uncheck it.
via Ubuntu Forums