Your receiver probably won't play the AAC files, since it seems normally the requirement is having iTunes installed (or an iPod) which is probably not possible on the receiver. I have an Onkyo with a similar streaming music feature, but it won't play iTunes AAC files directly.

The Airport Express (or Extreme) are simply routers. They help you share your internet connection to your wireless devices and help you network your stuff together. You still need something to access/play that data back from the network.