Hi
Hmmm.....Goodun! 
Check that your video does not actually have its own soundtrack. If it is silent there will still be a horizontal blue line running along the track.
If you try to drop your soundtrack onto this it will simply push the clip along.
If you have any stand alone sound clip, drag it from the media list and drop it onto a lower soundtrack (Soundtrack 2 or 3 for example)
From here you should be able to drag it right or left. If this works then mute the soundtrack belonging to the clip or click the track and unlink it; move it a little and then delete it. You can then drop your new sound clip onto the track to replace it.
Also check through the options tab to check what is set for when you add clips to the timeline.
Nat