Sound for New Mail

Already exists in the General tab and account-specific using about:config.

http://xnotifier.tobwithu.com/dp/node/2007

When you have an mp3 (or non-working wav), just open the file in an audio editor and (re)save as wav.

Step-by-step  :)...
- Custom sound for -all- new email...

X-Notifier, Options... General tab, "When new messages arrive", 'Play a sound', Advanced button...
...Browse to select a  .wav  file.

- Otherwise, as noted by Jeroen above...
...you can assign a custom sound per account... but when multiple accounts have new email, only the first sound plays.
(And sometimes, .wav files may need to be resaved by an audio editor to reinforce standard .wav file format/playability.)

When a suitable audio editor is not available on your computer, use an online converter. Googling 'mp3 to wav' will provide several options, for instance: http://audio.online-convert.com/convert-to-wav