“With more and more apps listening in at all times on your iOS device (Shazam, etc), short of deleting the app is there any way to control when this happens?”
Under iOS 7, yes. One of the features introduced with this version of iOS was the ability to limit apps’ access to the device’s microphone. When you first launch an app that wants to use the mic, you’ll see a dialogue box asking if you’re willing to let the app do this. Tap on OK and it now has the access it desires.
However, you don’t need to grant it this privilege forever and a day. It’s easily revoked by going to Settings > Privacy > Microphone and then toggling off the switches next to those apps you don’t want listening in.
Regrettably, this isn’t possible with earlier versions of iOS. If you’re terribly concerned about this, I suggest carrying a small roll of duct tape with you and placing a piece over the microphone hole (generally found next to the On/Off switch).
by Christopher Breen, Macworld