Play a Sound When New Mail Arrives
You can customize the "new message" sound when new mail arrives in Postbox. Go to:
Mac: Preferences > General
Windows: Tools > Options
and click the radio button next to Use the following sound file. Then, browse the computer for the sound that you'd like.
On Mac, the file must be an in .aiff or .wav format. You can find a good collection of .aiff system sounds in the following directory: Macintosh HD/System/Library/Sounds
On Windows, you must choose a .wav file.