Which iPad/OS X apps can be used to create music for a game

Discovr is a great app for this. You just enter the name of an artist, and then a list of related artists appear. A network graph appears with all of the artists. You can play previews, and watch videos. Pretty decent app.

This only hits your second requirement but, if you ask me, it's the more important of the two by a long shot: VLC. It can play just about anything you throw at it. It has support for play lists (both building and saving them) but it does not do any organization of your media. It is strictly a player.