Well, here's a pretty simple answer. Because all music in the NG Audio Portal is under this Creative Commons license (it says so on every song's page), you cannot use the music for commercial projects. So because you'll be making money off of the games, you'll have to contact and make arrangements with the author to use a song or create original music for you. And of course, many artists work hard on what they do, so musicians are going to want some kind of cut for the music that is used.
It's a quick answer, but someone might give a more detailed one or maybe correct anything I said.