location of mail notification sound?

[Ned Lilly]

Sorry if this is a FAQ, haven't seen it anywhere...

I know there isn't a button (yet) to select the sound that's played 
when you get new mail (I'm using 0.81 for Linux).  Can someone point 
me to the location of the file?  Right now it's a default beep.

Please reply via email if possible.

Thanks,
Ned