Hey. I know a thing or two about copyright law (well, in truth, that's a bit of an understatement).
What Doctor Octoroc has said above is pretty spot on, and backed up with real world experience, so you should listen to them. Some specific thoughts from me:
a) 7 out of 10 tunes I want to cover are demoscene music, which in general is on public domain licence. I don't think I need any additional licences for these.
As Doctor Octoroc said, if you know for a fact that something is in the public domain, then you can do what you want. The chance of this actually being the case is rare though, unless the authors' have specifically released their material under something like the CC0. Just underlining this point.
b) because there's probably no way to acquire licences for retro games music (apart from contacting the authors themselves?),
There's a few misconceptions here. Firstly, an inability to get a license doesn't mean the material isn't protected by copyright (though this varies by jurisdiction). Secondly, the authors won't necessarily own the rights to the material anyway. It could well be that the companies that produced the games own the rights.
c) because - in the end - it's generally non-commercial. I know it may be not all that important to some people but I'm counting on some common sense of the original authors. wink
Again, a few things. Firstly, as soon as you put it on a streaming platform, that's a commercial use. It doesn't matter if Spotify will basically never pay you anything - it's the nature of the use that counts, not the end product. A failed business is still a business. Secondly, non-commercial use doesn't allow you to use material without permission by default. There are concepts of 'fair use' and 'fair dealing' which do take into account the nature of unauthorised use, but that wouldn't apply in this case, for a whole variety of reasons.
Also, there's other considerations here. Again, the authors may not own the rights to the material. The original development company (or their modern form) may. Secondly, lots of folk won't be happy or view things the same way you do in relation to common sense if you haven't spoken to them beforehand. Third, it is pretty trivial for them to have the release pulled from distribution if they are unhappy. Finally, you need to make a declaration to your distributor and/or the platforms you release on that you have the necessary rights to use the material. If you don't, and they find out, then many of these distributors don't give second chances, and you could lose your ability to upload other material in future (this is not going to be as strict on something like Bandcamp, but still).
I hope that helps! In this case, since you're talking about artists who are not in the major commercial leagues, I would reach out to them and explain your project. Explain why you love the music and want to make a tribute, offer to credit them etc, and gain their blessing. They might tell you they don't hold the rights to grant, they might tell you no, or they might tell you yes... but if you can't get some kind of confirmation from them, it would be unwise to go ahead and release the covers on a commercial platform. Litigation may be unlikely (unless Nintendo are involved!), but it's poor form if nothing else.
