I just solved this issue myself and thought I’d add to this old thread for anyone else looking for answers. What I’ve found with FCP7 is that often when opening and working on old projects that have been moved a few files get totally disconnected and the link is corrupted. What I mean is that a file (usually a graphic or audio track) just comes up blank in the viewer. If you try and reconnect it you get a message saying “Cannot Find File”. If you try and view the file in the viewer it says a similar thing. The file is so disconnected that it doesn’t even come up with the ‘missing media’ screen in the viewer, let alone show that its disconnected in the timeline.
My ‘Not Found’ error while rendering was from an audio track that was disconnected and ‘corrupted’ like that. I tried all the previously listed fixes but none worked in my case. What made it tricky for me to find was that the disconnected audio file doesn’t show up disconnected (white) in the timeline (it looks completely normal (green) if you have wave forms turned off). To add to the trickiness the ‘Not Found’ message was popping up 5 seconds or so before the audio track was to be played. Very confusing and hard to detect!!
Once I realized this file was kaput (well the link in the project timeline was) I just found the file on the hard drive, re-imported it and copy/pasted its content back on the clip. Whallah! back in business. You could also delete the track from the edit and cut it back in, but copy/pasting the content was faster.
So if anyone out there is chasing this error then have a look at clips which are not only in the error zone, but also proceeding it. Open each clip in the viewer and confirm it can play and is linked properly and the timeline link/edit to the file is not corrupted.
hope that helps someone out one day!
Jono.