What are the terms of use for keygen music tracks on this site?
I want to use one of them in a smartphone game I'm developing and are going to sell. Is that ok, not ok or ok if I credit the creator? I understand if the creator don't want me to earn money for something that was intended to be free but maybe it can be published with an apache license or something.

If you give a credit to the original authors and the project is not commercial then yes, in most cases (99,9% i think). For the really commercial projects it's better to discuss it with authors.
See the comments for instruments and samples, sometimes you can find author's name and/or contacts. If not search the same on somewhere like modarchive.org to find out who is the author.

Ok thanks!