If, how and where can I specify the sound application I want Thunderbird to use for the new mail sound notification?

The sound works, but it sounds terrible and I don't here a sound when playing mp3's. I use software sound mixing with ALSA and would like to specify the application TB should use so I can mix the new mail sound with any other soundfile I might be playing at the time of new mail arrival.

I can't find an option in the user.js/prefs.js, so do you know how?

(also posted on mozillazine, but it's all Windows down there...)