Rated 5 out of 5 stars

Very good addon, i'd like to have posibility to turn off sound in options

This review is for a previous version of the add-on (0.0.4a.20090907). 

about:config pref: extensions.alertbox.audioalert

Currently you can set preference `extensions.alertbox.audioalert` to false to disable audio alert.
More advanced controls options will be available in near future.