You go to 'Share' up above the Preview screen. Click that. And then to the right of the preview screen, there are a number of icons in two parallel rows. The first is 'Create Video File'. Click that. Then in the dialogue box which appears, choose what format you are after. If the new single video is to have the same properties as the clips in the project, then choose 'Same As First Clip'.
The audio recording function in VS is there, but a bit simple. Many of us find it better to use a dedicated audio editing program -- such as the freeware Audacity -- which gives you greater flexibility. Once your voiceover is recorded, then right click on one of the audio tracks down at the bottom and choose Insert Audio.
You can do the audio before rendering the project out to a single video. That is usually better as with most video formats, the more renderings you do, the greater will be any degradation for that particular format.