HI
Note that when you move a clip by dragging it along the Video track, BOTH THE CLIP AS WELL AS ITS SOUND TRACK will highlight (turn blue).
If you have unlinked them then only the selected track will highlight and it will only be THAT track which will move.
If you have inadvertently unlinked them, right click one of the clips (the video or the sound) and you should see the option to restore the linkage. You will be able to do this even if you have already moved the clip. Click on and close any gaps this produces.
Check also that the clip audio (if unlinked) has not found its way to a lower Audio track as it may not be visible.....preview the "gap" and see if the sound is there.
The Audio track should NOT disappear unless you unlink and specifically delete it
Nat