Thanks for the quick responses.
Miggy: Yes, I am trying to create one video stitched together from multiple clips, with one audio track overlain. Your suggestion about splitting the track at a certain point sounds as if that could be the issue as normally the song continues after the end of the last clip - how would I know if the track was split?
Borate: Am I right in assuming that to lock the track you click on track and select “lock track” under the menu tab? When I do this all options are greyed out and I am unable to select anything?