I found this rare issue on my iPhone 3GS. I have turn on mute button to prevent any apps making alert sound. While I am on my GodFinger game screen, I received a call from my friend. After the call is hang up, the game resume(that is what multitasking should be) to original state and I can continue my game.

But the sound in the game automatically turn on even I have turn it off manually before I start the game. I suspect this is an issue on the apps which it doesn’t cater the resume state properly. This is not a big issue and it can be fixed.