Lesson on Audacity exporting. There is a caveat when exporting your audio, especially if you divided a narration in clips, each clip representing different phrases. This is the original initial vacuum:

Of course, you would assume that vacuum between the time 0 and the start of the clip would be interpreted as silence when exporting. Unfortunately, it does not happen, so the audio is exported as if it started from the beginning of the clip(and not the beginning of the track). To fix this nuisance, allow the selection to snap to the beginning of the clip, and the left side of the selection to snap to the beginning of the track:

Then the silence generator applied to the previous selection yields:

Now if you export the track(by selecting the whole track and clicking on 'export selection'), the audio file will have the initial silence. We can still join the clips(by selecting around the joint and pressing ctrl+j):

To finally have the initial silence and clips fused: