Okay, since PT keeps thinking there is a video track, there must be some remnant hiding somewhere. Here are some things to try:
1-Open the session folder and look for a video file. If you find one(or more) move the file(s) out and stash it somewhere else(but NOT on the desktop).
2-Open the session, assuming no video track is visible, select all the tracks and then Export them as a new session(include audio files) and save. Open this new session and see if the video engine stays off.
3-Create a blank session with video engine off. Import Session Data from the original and pay close attention as you select tracks to import(in case there is a hidden video track on the Tracks list).
Fingers crossed that one of these helps
