Same thing happen to me. Here's how I fixed it.
The icon is there. Its just not where it used to be. Since the IOS 7 update, they moved it.
You just have to decide if want to play the audio on your iDevice (iPhone, IPad) or airplay_device. If apply tv (or other airplay device) then you have switch the device to output out of applytv instead of iDevice. (like switching speakers)
Looking at your home scrren, put your finger on the bottom of the screen and swipe up.
That beings up menu and there is the airplay icon. Switch from iDevice ot Airplay Device.
Thats it. I have apply TV and that resolved my issue. Now i can play music from my ipad and iphone on my ApplyTv.