Topic refers to:
extensions.xnotifier.accounts.[HOST#YOUR_ID].sound [and set it the full path to your WAV sound file...]
1) Type 'about:config' in the location bar.
2) Look for HOST account with 'enabled' [Right-click] Copy Name
3) [Right-click] New => String
4) [Right-click] => [Paste] and replace 'enabled' with 'sound'
5) [Click] OK
6) [copy/paste/typing] See below Window or Macintosh with explain.