There are a few ways to do this. One way is:
1) Click on the clip in your timeline.
2) Go to the effects control tab in the Monitor window (or Window > Effects Controls)
3) Expand Volume
4) Expand Level
5) At the point where you want the volume to start fading add a keyframe by clicking on the diamond shape in the middle of two arrows (when yoi hover over it, it displays "add/remove keyframes")
6) move along the timeline to the point where you want the volume to be zero, add a keyframe and move the level slider as far to the left as posible.
7) add a keyframe where you want the volume to start increasing again

add a keyframe where you want the volume to be back to normal again and set the level to 0db