From: DeskNow Knowledge Base
http://www.desknow.com/kb/

How do I change the sound that is played when a new email arrives?
http://www.desknow.com/kb/idx/5/177/article/

The sound is stored in the file

C:\Program Files\DeskNow\desknow\3.2.9themes\sounds\alert.wav (on Windows)

/var/desknow/desknow/3.2.9themes/sounds/alert.wav (Linux/Unix/Mac).

(replace 3.2.9 in the above example with the the version number that you're using).

Simply replace the alert.wav file with one of your preference, and make sure to clear the browser's cache to ensure that the browser reloads the file!