Mac: disable the "boop" sound played when search results are not found

Solution 1:

This is the default "alert" sound used as the audible bell.

Disable alert sounds altogether

You can generally disable it by lowering the volume — Note that unchecking "Play user interface sound effects" is not the same but it only disables sound effects from Finder, etc. The alert will still go.

Also, as far as I'm concerned, you can't disable it just for some apps.

Alerts in Firefox

Firefox seems to have special settings that you can access in about:config. You can disable those, although I personally didn't hear any sounds when searching in Firefox (4.0.1), but this could be OS specific?