If the audio is inside a video clip, then you need to extract the audio first. Look at "Advanced" menu, it should be listed under that. Then you can highlight the audio, move the cliphead to where you want to start cutting out and go to "Edit -> Cut selected waveform at cliphead" and then move the cliphead to where you want stop cutting and do the same Edit/Cut procedure. Then highlight the mid-section that you want to get rid of and use the pull-down menu on the left bottom corner to push the volume level to 0%.
If you want to fade in and out of the silent section, then you can go View->Show Volume level or something like that. This will show you an average volume level line on each clip. You can click on that line reduce the volume level gradually. It takes some playing with it, but it is fairly easy.