How to programmatically sense the iPhone mute switch?

Thanks, JPM. Indeed, the link you provide leads to the correct answer (eventually. ;) For completeness (because S.O. should be a source of QUICK answers! )...

// "Ambient" makes it respect the mute switch// Must call this once to init sessionif (!gAudioSessionInited){    AudioSessionInterruptionListener    inInterruptionListener = NULL;    OSStatus    error;    if ((error = AudioSessionInitialize (NULL, NULL, inInterruptionListener, NULL)))    {        NSLog(@"*** Error *** error in AudioSessionInitialize: %d.", error);    }    else    {        gAudioSessionInited = YES;    }}SInt32  ambient = kAudioSessionCategory_AmbientSound;if (AudioSessionSetProperty (kAudioSessionProperty_AudioCategory, sizeof (ambient), &ambient)){    NSLog(@"*** Error *** could not set Session property to ambient.");}

I answered a similar question here (link). The relevant code:

 -(BOOL)silenced {     #if TARGET_IPHONE_SIMULATOR         // return NO in simulator. Code causes crashes for some reason.         return NO;     #endif    CFStringRef state;    UInt32 propertySize = sizeof(CFStringRef);    AudioSessionInitialize(NULL, NULL, NULL, NULL);    AudioSessionGetProperty(kAudioSessionProperty_AudioRoute, &propertySize, &state);    if(CFStringGetLength(state) > 0)            return NO;    else            return YES;    }

Some of the code in other answers (including the accepted answer) may not work if you aren't in the ambient mode, where the mute switch is respected.

I wrote the routine below to switch to ambient, read the switch, and then return to the settings I need in my app.

-(BOOL)muteSwitchEnabled {#if TARGET_IPHONE_SIMULATOR    // set to NO in simulator. Code causes crashes for some reason.    return NO;#endif// go back to Ambient to detect the switchAVAudioSession* sharedSession = [AVAudioSession sharedInstance];[sharedSession setCategory:AVAudioSessionCategoryAmbient error:nil];CFStringRef state;UInt32 propertySize = sizeof(CFStringRef);AudioSessionInitialize(NULL, NULL, NULL, NULL);AudioSessionGetProperty(kAudioSessionProperty_AudioRoute, &propertySize, &state);BOOL muteSwitch = (CFStringGetLength(state) <= 0);NSLog(@"Mute switch: %d",muteSwitch);// code below here is just restoring my own audio state, YMMV_hasMicrophone = [sharedSession inputIsAvailable];NSError* setCategoryError = nil;if (_hasMicrophone) {    [sharedSession setCategory: AVAudioSessionCategoryPlayAndRecord error: &setCategoryError];    // By default PlayAndRecord plays out over the internal speaker.  We want the external speakers, thanks.    UInt32 ASRoute = kAudioSessionOverrideAudioRoute_Speaker;    AudioSessionSetProperty (kAudioSessionProperty_OverrideAudioRoute,                             sizeof (ASRoute),                             &ASRoute                             );}else    // Devices with no mike don't support PlayAndRecord - we don't get playback, so use just playback as we don't have a microphone anyway    [sharedSession setCategory: AVAudioSessionCategoryPlayback error: &setCategoryError];if (setCategoryError)    NSLog(@"Error setting audio category! %@", setCategoryError);return muteSwitch;}