When you click on "Browse" you should go to your AirNav RadarBox/Data folder.
You should find at least one file there with a .WAV extension - I can't remember its filename - possibly Reminder.WAV, because I moved several other sound files in there as well to have some variety.
Chose the .WAV file, click Open, and you should be in business.
RadarBox will play any .WAV file, so you can bring in files from other places for fun.
Hope that helps
Rod