First, need to discuss surround sound. When your receiver is displaying 5 channel stereo it is playing the same sound from all 5 speakers. This is a mode of the receiver intended for plain stereo signals. When your receiver displays Dolby D it will play the specially encoded audio from the ATV which actually plays different sounds from the different speakers to give a 3d effect. This second mode is what is known as surround sound commonly.
If you want to force your receiver to play 5 channel stereo instead it can be done. Please post a picture of the back of your receiver so I can see all your connections to give specific directions.
On the Airplay issue, please post the following information to begin the discussion:
What specifically are your trying to do with Airplay?
What specific device are you trying to Airplay from?
What is the internet connection that your ATV and the Airplay device are connected to?