Some users have reported an issue in which the AirPlay icon (for streaming music and video to AirPort Express base stations and Apple TVs) is suddenly missing from various apps on iPod touch, iPad or iPhone.
There’s a simple fix for this issue, as follows:
On your iPad, iPhone or iPod touch, navigate to Settings > General > International > Language and change to a different language (e.g. if you are using English, switch to French). The device will go through a language switch/reset process, and your AirPlay icon should be restored.
Next navigate back to Settings > General > International > Language (translated into the new language, of course) and switch back to your original language.