I don't know if your are targeting Iphone but seems this behaviour must be specified by users/players in general "settings" and not by C2 dev ! ... so if a user want his phone to be locked after 3 minutes in all apps when doing nothing you can't prevent him to do so ! ;-p
I'm talking about a droid but for iPhone that is not true, if you was to watch a movie on it or live tv with tvcatchup, it wouldn't lock regardless of settings, in my app, I have a few different frames that play along with audio, your not meant to touch just watch and listen, but the phone eventually locks :(. Really need a way to stop this or my app is kinda pointless.