You can click anywhere where the sound is located then at the bottom click on edit button, and go to the part where you want to silent it, dragging the both squares down, and adding another square where it starts getting silent.

I know another way thought, get a free trial of Goldwave.

http://www.download.com/GoldWave/3000-2170_4-10001099.html?part=dl-GoldWave&subj=dl&tag=button&cdlpid=10001099

Then just remove the part you don't want.