When you click activate button of this software it's starting to produce to mosquito repellent ultra sound.

 
The sound range  which is this program produce beyond the audiable range of humans. That's why you can't hear it. Pests and other animals can through  hear the very same frequency. This sound produces an undesirable effect on mosquitos and other pests thus restrain them from flying. 
This ultra sound is completely safe from damaging your hearing