You don't need to. With Audio Session you can decide how the audio should behave.
From the Audio Session Programming Guide:
With the audio session interface, you
specify aspects of your application’s
audio behavior and configure it to
live harmoniously within the iPhone
audio environment. You start by asking
yourself questions such as these:
Do you want your audio to be silenced by the Ring/Silent switch?
The answer is probably “yes” if audio
is not essential to using your
application successfully. (Users will
appreciate being able to run your game
in a meeting with no one the wiser.)
Do you want iPod audio to continue playing when your audio
starts? This could be appropriate for
a virtual piano, letting users play
along to songs in their libraries.
You’d want iPod audio to stop,
however, for a streaming radio
application.
You probably want this:
UInt32 sessionCategory = kAudioSessionCategory_SoloAmbientSound;
AudioSessionSetProperty (
kAudioSessionProperty_AudioCategory,
sizeof (sessionCategory),
&sessionCategory
);
For more behaviour types, check the Audio Session Categories, or read the entire Audio Session Programming Guide.