Rated 1 out of 5 stars

Please, Please, Please support .aiff sounds so this can be used on Mac OS X without converting existing system sounds.

This review is for a previous version of the add-on (1.1.4). 

Feel difficult

As far as I know, the underlaying nsISound interface only supports WAV files now. That's a reason why current sound add-ons only support WAVs.
To add other types available would be difficult to author (me, yes).
A compromise solution to your case, next version I'll try supporting system sounds first, like Navigational Sounds does (https://addons.mozilla.org/firefox/addon/1030).