ones you make revenue from your game by selling the game with his song inside then you will have to give the guy a % or buy a license from him, but... if you game is free, and you have only adds and in-game monetary system that doesn't require for the game to be bought. then your partially safe.
but you still use the song in a potential making cash game, so he might contact you first then if you dont answer will sue you.
and yes if you did used some people songs, well its not always necessarily for u to add the credit line for the song, unless he specifically asks for. so you'd be impressed how easy is to get a song to be licensed. and i wold suggest you to contact the sellers or the author, to get a license from him, and will make your life much easier later, if you contact let him know ur a startup developer and u need a cheap license of the song and he will make you a list of prices or tell u if he will charge u anything. i hope it helps, you have to contact the author or the people that has the publishing rights, which is stated in the song download page.