Set your application's audio-session category to AVAudioSessionCategoryAmbient
by adding the following code to your app delegate's applicationDidFinishLaunchingWithOptions:
method:
NSError *sessionError = nil;
[[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryAmbient error:&sessionError];
[[AVAudioSession sharedInstance] setActive:YES error:&sessionError];
[[AVAudioSession sharedInstance] setDelegate:self];
From apple's doc:
This category allows audio from the iPod, Safari, and other built-in applications to play while your application is playing audio.