Do this by selecting Generate > Silence from the menus at the top of the screen. Here I have selected the very start of the vocal track, as I want to insert silence in that track to allow the music to come in before the voice begins speaking: When the voice track is selected you will see the control set of that track go a darker color, and at the point of selection itself your cursor becomes a pointing hand. We now click in the voice track where we want to insert silence.

Here I have loaded an audio track (in mono), and a stereo music track beneath it:

This would work in a radio advertisement or introduction for a radio show or podcast. If we take the example of a two track project with one track as music and the other track as a voice track, we can imagine that we might want to insert a silence in the voice track for several seconds to allow us to 'fade in' the music. You may want to add a silence to a track for several reasons. We will cover each of these operations below. Some of the more common include adding silence, trimming audio, advanced cutting and pasting, splitting and joining tracks, and using panning. There are many more advanced steps for editing that you can carry out with Audacity.

Audacity: AdvancedEditing Advanced Editing and Mixing

Mixing in audacity