Is it somehow possible to combine the audio sequence of a youtube video with another video? The user enters a youtube url and uploads a local video. Now I want to combine his video with the audio sequence of the youtube url. Is that legal? Is it possible? Data API? How? Do I need to download the youtube video to my server first? Thank you.
Technically, you can probably do this. There are services out there which strips the audio from existing youtube-videos, so the tech is already available. The rest is basically adding the audio track to the uploaded video (after stripping out audio already present, I assume) and then showing the combined product. This, however, is not a very straight forward service, and will probably need quite a bit of server-side coding - not in PHP or similar, but with C or JAVA or some other executable code. As for the legality, no, it won't be legal. Unless you get the permission from every single owner of the original audio-tracks from the Youtube-videos beforehand, which is probably not very likely.