Even if he did use MC for images, which he is not, why would opening an image create a Zone?
MC can play both Images and Music at the same time. It is a nice Slideshow effect. It is easy to use. If you start playing some music, and then while it is playing, you "play" a view full of pictures, it keeps the music going and shows the images in a nice slideshow automatically. The way it does this "behind the scenes" is that if playback is active, and you play a view full of image files, it auto-creates this special Images zone and plays the files there.
The Images Zone is a special zone that is only auto-created like this, so it doesn't show up in the normal Zone definition section. It is "destroyed" when you stop the image playback.
This is what is happening to him. When you play that second set of files, MC thinks it is playing some pictures, and creates the Images zone and sends playback there. The playback is distorted because the zone isn't set up right (they are supposed to be photos, after all).
The Overview screen is accessed under Playing Now in the tree (where your Zones are listed, if you have any, and you can select from the different Playing Now lists). It shows the playlists for all of your active zones at once (it is handy). It doesn't "pop up" on it's own in any circumstance I see, unless you select it in the Tree.
One thing that is confusing is how is he playing these files? What you are saying seems to be contradictory. You say it "only happens when the Overview screen is showing". But if the Overview screen is showing, how is he browsing through his files and choosing something to play? Now, if what you mean is that "when this happens, the music overlaps, is distorted, and the Overview screen appears suddenly" then it kinda makes sense that the Overview screen might appear. MC is horribly confused. It tried to play some pictures fullscreen but they were music files (again, they are
supposed to be photos), so it freaks out and shows the Overview playing now screen because it doesn't know what else to do but crash.
As far as why this is happening, I'm not sure....
Can you describe his View that he is using to play these files? You mentioned the folder.jpgs. Maybe he is somehow playing
those files, and not the audio files themselves? I don't know, I'm guessing. Tell me how the view is constructed (what categories it uses, what it looks like, and what filters it has), and how he is managing to both be on the Overview screen and selecting files for playback at the same time.