So I'm a sound designer and I want to make a demo to show off what I can do.
As something a little different, I had the idea of capturing gameplay footage from a well-known game and completely replacing the audio track with my one of my own creation (sfx & music both).
Aside from whether it will really matter and whether it's a good idea or not (yes, I'm aware that this isn't how sound works in real-time interactive media), is it actually legal?
Cheers
It's completely fine - be
It's completely fine - be sure to acknowledge the original developers and clearly point out that you didn't work on the original game, and that you've purely used their footage as a means of demonstrating your own sound design ability. Keep it for private use (send it to developers as part of a resume, but I wouldn't host it on a public website) and you're all good.
1 - Acknowledge the developers
2 - Clearly point out that you had no associating with the game
3 - Keep it private
Gameplay footage use
You know, I've never heard of a case where a developer had an issue with the use of modified gameplay footage (fan-made or otherwise). For the most part, they're pretty happy that their game is getting extra exposure.
The only times I've seen developers get a miffed is when people upload entire walkthroughs of games right after release date (Konami got a guy suspended on youTube after he put up the entire Metal Gear Solid 4 walkthrough for example), or any footage when a street date is broken (Grand Theft Auto 4).
You should be alright, as long as you credit yourself for the sound + music only, and, heaven forbid, don't start charging cash for it.
I'm no legal guru, but simply putting music to gameplay is not dissimilar to making a music video, which crazy fans do for animes all the time.
Using someone elses music is more contentious because the musical industry is sue-happy. This is probably where any of your worries may have stemmed from.
And you're not really modifying to profit, but promote. All seems fine to me.