Hmmm, that's not very user friendly.... I also read that soon Mozilla is handing over TB development to the community, so we'll have to see if that's going to improve or decline things...

Because I can't find a solution too directly, I hope you can get along with another wav file in the near future...
=> Is it possible for TB to use other files than WAV? => maybe converting you sound file works?
Other possibility: add a new user account to the system, and do a test there (although I know it's not very easy to "test" a mail client) with that specific WAV file....
=> but I think that won't make any difference...
And too: if you upload the WAV-file somewhere (minus.com, dropbox, ....), we can test it on our system too if needed....
More, I can't think of - maybe someone else with some wonderful idea?