From MozillaZine Knowledge Base
Find As You Type (or Type Ahead Find as it was originally called) allows you to start finding text on the page immediately by simply typing the text you’re trying to find. When typed characters fail to match anything on the page, it can be configured to play an alert sound.
Possible values and their effects
If the typed character doesn’t match anything on the page (and the user has typed at least one character previously), play an alert sound. (Default)
Never play an alert sound. (This is the same as setting accessibility.typeaheadfind.soundURL to an empty string.)
A checkbox labeled “Play a sound when typed text isn’t found” is located under “Edit → Preferences → Advanced → Keyboard Navigation → Find As You Type”.
First checked in
Has an effect in