Here is what I did.
I cut a clip, and then deleted the audio in that clip.
I cut another clip that contained suitable shop background noise and copied the audio out of that clip.
The audio clip was not long enough to fill the deleted audio in the first clip.
I "stretched" the end of the short audio clip to fill the gap.
The clip plays as I wish it to but I get numbers in the upper left hand corners of the clips????????
That is just telling you that the audio is out of sync with the video. Ignore it because that is what you want.
I did link the audio to the video, still have that number tab?
It's not that they are not linked, but not synced:
Linked means that moving one will move the other.
Synced means that the audio is playing at exactly the same time as the video at the time that they were recorded.