There is a problem with ios 7 and the older Airport Express/g.
I have two Macs running ML, an iPhone 4S and iPad 3 both on ios 7. I can play airtunes on external speakers through iTunes. The Airplay icon is there and I can select Airport Express. When I try to connect with the iPhone or iPad by swiping up from the bottom and selecting the AE icon it will allow me to select it BUT reverts back to the Airplay icon within a few seconds and there is no music played. Swiping up again has the same effect. I have factory reset the AE but nothing fixes the problem. This only became a problem the day after updating from 6.1.4 to 7