Due to a bug in GDM there currently is now way of preventing GDM from playing a sound when you login to your Fedora 11 box. This is especially annoying on notebooks, e.g. when you’re sitting in a meeting or in a (quiet) public place like a library.
The only workaround (a bit dirty, granted) is to delete or rename the sound file:
# cd /usr/share/sounds/freedesktop/stereo/ # mv bell.ogg bell.ogg-bak
Enjoy reclaimed silence!
Update: There is a workaround in bugzilla that gets by without deleting or renaming any files.