.Mov files with audio but no video in QuickTime

I'm assuming the .mov file contains the clip somehow, but the video for the clip isn't playing. I'm not sure how to actually tell if it's on the same video track because file can't be opened in iMovie, only QuickTime.

You know what they say about "assumptions..." In any case, while I too believe the problem is more likely associated with the use of multiple video tracks (and/or layer compositing), it's best to confirm that the video data does, in fact, actually exist and in which compression format. As to the determination of track use, any video utility that can read the track ID associated with any particular data track should be of help here. I still tend to prefer the QT 7 Pro "Properties" window since it displays both the track and layer use for composited video data as depicted below:

If you expand the screenshot above, you can readily see that Video Track 1 (first video segment) and Video Track 2 (second video segment) have been allocated to file track IDs 1 and 3 respectively. Further, you will note the second video segment merged into this file has no associated audio track (time segment 7:42.42 thru 9:10.58)—meaning no audio will be present during playback of this interval. While different than your own problem where no video is being played, it does demonstrate one possible explanation for your lack of video playback. The more likely explanation in your case would be that the last video segment is assigned to a different video track which is being ignored during playback in modern media players like QTX but remain playback compatible with legacy media players like QT7 which can handle sequential or simultaneous playback of up to 99 tracks of data.

From what I gather these .mov files are automatically created when you create an iMovie project (.iMovieProj file) in iMovie 6 HD. I can access my movies by going through the "shared movies" folder (which not all my projects contain) and then either going through a Garageband or iDVD folder, which contain the .mov file. I never purposefully created the .mov file in iDVD, however it seems to contain the finished product of my movie, occasionally omitting video.

Exactly! Prior to the use of "by reference non-linear editing" concepts used currently, iMovie 6 HD used "reference" MOV files that point to "physical" data stored in source files and intermediate files created during the video editing process. Basically, these "reference" files represent the current "edited video timeline" telling which segments of which files go in what order for iMovie 6 HD export, sharing, GarageBand editing/chaptering, iDVD burning, etc. Such files are created as needed for specific target use. Since these files are relatively small compared to the physical data they represent, they provide a convenient way to pass data between apps/routines. Basically, they should continue to work properly in legacy QT apps as long as the targeted physical data paths do not become orphaned (by moving files around) or the source/intermediate data files are not corrupted/deleted. Unfortunately, without more information regarding the files themselves, it is difficult to diagnose specific issues and dependent solutions since codec support requirements have changed since iMovie 6 HD was in use and following the release of Mavericks and its introduction of new independent QTX structure embeds which continue to be used.

It's frustrating because it would be a lot easier to use these .mov files rather than dragging all my clips BACK into iMovie HD to reassemble a project I made a decade ago

It might have been easiest to incrementally update the iMovie project files to match updates to the iMove app and Mac operating system over the last decade. You may be facing more than a single issue here.

Updated information if this helps: I can access a "media" folder in my projects that contains the entirety of clips used in the project. I imagine I could just reassemble the project, but I'd rather just use the conveniently already created .mov file. Is there a way to "update" the .mov file with all the clips? For instance, one of my project/movies had 5 scenes, and the .mov file shows 4 of them with video and audio, and all 5 with audio, leaving one clip without video.

Solution?

Does the MOV file actually contain the 5th video track segment? If so, is it on the same video track layer as the other video track segments? If so, does this video track segment contain data in the same video compression format as the other video tracks?

Basically, it's unclear at this point if the MOV file to which you refer is a reference file representing the current project timeline, a standalone file previously exported at specific settings for a particular use, or the saved merging of two exported project files to different video track layers which modern players like QTX don't like to play but remain QT7 playback compatible.