Hi Tygre,
The URL cited is definitely a valid option that is "allowed and smiled upon". Hopefully you won't call it millions of times per second ... As for credit, you could say it's coming from AMP ? not much more to say, I guess, aside crediting the musician(s) behind any given music, of course

Regarding a "best practice", I know nothing about API and how it should behave. Calling an ID with downmod.php means that you know on your side, somehow, the metadata that comes with it. This URL gives nothing more than the file itself corresponding to the ID given. I have no idea how you would know the author(s) on your own nor how you would maintain this information. But then again, maybe you do or you don't need this information

Anything could be done on our side. Definitely. But I would need material, or best practices for that kind of thing. I understand it could mean "searching" at some point and I most definitely don't want to search for '*' .. ever

So, what do you have in mind, exactly ? I don't know your program so you'll have to take that into account in your reply

Cheers
Sylvain