From MozillaZine Knowledge Base
This article describes the options to mute sounds coming from Firefox and Mozilla Suite.
Sounds in webpages
Some webpages have sounds that might distract you from surfing or listening to music. The solution depends on how the website is using those sounds.
If the sounds are coming from an ad, they're likely from the Flash plugin. In this case, you can use Flashmute, a Windows-only utility to mute Flash, or Flashblock, an extension that disables Flash until you click on it.
Other options include:
Sounds in browser
If you use Find As You Type and it doesn't find the word, it will play an alert sound. To disable this sound in Firefox, type about:config in the Location Bar and press enter, locate accessibility.typeaheadfind.enablesound and set it to false by double-clicking on the value. To disable this sound in the Mozilla Suite, uncheck the option "Play a sound when typed text isn't found" located under Edit -> Preferences -> Advanced -> Keyboard Navigation -> Find As You Type.