Rated 4 out of 5 stars

Been using Mailbox Alert for some time, and it's always worked great. But I've always wanted to play an individual wav file per sender or per subject line (like I used to do with Eudora filters). Since Thunderbird's filters can't do it, I'm now using a combination of Mailbox Alert and smcWavPlayer, which is a utility that I wrote that works along with Mailbox Alert. It allows you to specify specific wav files to be played upon receipt of an e-mail from a specific sender or with a specific subject line.

http://home.snc.edu/scott.crevier/tools/smcwavplayer/

This review is for a previous version of the add-on (0.13.1).