I'm making a Web app (at chirpid.com) for the iPhone that plays audio files for cricket chirp identification. The user can start and stop the audio by tapping screen buttons. But if the user taps the home button while a sound file is playing, it continues to play in the background (an iOS4 feature). I want to stop the audio in this case. Is there an event or property that I can use via Javascript in Safari to determine when I have been put into the background?