You may have already figured this out, but an easy way to delete sections of voiceovers is to firstly split the audio clip at each end of the unwanted portion.
To split the audio clip, move the skimmer/playhead to the start of the unwanted section then right-click (or Control-click). From the pop-up menu, select Split Clip. The full audio clip will now appear as two separate clips. In the second section, move the playhead to the end of the unwanted portion then split the clip again. Now you will have three sections. Delete the unwanted middle section by simply pressing the Delete key on the keyboard.
If necessary you may have to move the two remaining sections into their appropriate positions under the video thumbnails. To fine tune the end and start points of these clips, drag the ends in or out (hover the mouse pointer over the ends until you see a left/right arrow appear, then click and drag inwards or outwards to shorten or lengthen the clips).