Ohhhh okay, that makes a bit more sense. Well, you're right, it is sort of a grey area; Personally, I've only received positive feedback. For example, the audio director at Klei Entertainment even commented on my Don't Starve reel and gave me some feedback on my compositions.
The reason for that is most game studios are generally open to people doing let's plays and walkthroughs of their games on Youtube because it is free publicity to them. The grey area is that there are some studios that don't allow these types of videos, mainly because of Youtube monetization; Nintendo is an example. I wouldn't be able to do a Nintendo game in my series because they have been going after people in the past for showing Nintendo-trademarked content and earning money off of it. You would most likely be able to display if for free, however.
For film, however, it could be a different story, I'm not sure. With these mock ups, we as composers own our music. We don't own the footage that we compose over or are simply inspired by. Going along with the game guidelines, you should be fine using the footage with your music on a portfolio, just don't go selling both of them together on a DVD or something claiming that you do in fact own the footage. Sure, if you happen to make a kick-ass song and want to sell it without the video, you are more than welcome to because it's yours. In my experience (even as a web designer), going along with your photography statement, the reason that photographers go after designers and Youtube slideshow creators is because they never received appropriate credit.
If I were you, I would post it because a.) you did a great job on it, and b.) what's the worst that could happen? They won't sue you for showing it. If anything, they will simply ask you to remove it. Just be sure to state that the video does not belong to you, which you did.