No Mutants Allowed

Everything older than 70 years is free-for-all, if I understand correctly. However, it might still get complicated with orchestral performances of a lot of older music, since they still require their credits.

Yes its something like however after looking for awhile its looking alittle bit complicated. As you have to take into account who's recorded it etc. But I think I've found a good website that listed their recording date so is very helpful. But yeah you can't just take stuff of YouTube etc

I guess one thing you could do is to carefully browse ambient tracks on Soundcloud, and litterally e-mail every artist you like, one by one, and humbly ask for permission. Many artists don't mind helping out, as long as you ask first. It could be worth a try

(they would of course want to be credited, a mention in the "end credits" or something)