A friend of mine just hates those little beeping error notices. I don’t know about you but for some, it’s like a rusty nail on a blackboard. With Firefox, you may find one of those beeping sounds when you the Find feature doesn’t get a match for your word or phrase string. It’s accompanied by the Find textbox going red and a warning sign indicating “Phrase not found.”
If you’re annoyed by that beeping sound then this little tweak is for you:
- Access the about:config page.
- Find the accessibility.typeaheadfind.enablesound preference name
- Double-click on it to change the value to false
- The changes take effect instantly

I use Firefox 3.5.9 on Linux and the above tweak does not work.
Here I found a working solution:
http://kb.mozillazine.org/Accessibility.typeaheadfind.soundURL
search for “soundurl” in about:config and set the string “beep” to an empty string.