How to disable iOS System Sounds
[[AVSystemController sharedAVSystemController] setVolumeTo:0 forCategory:@"Ringtone"];

- (void) setSystemVolumeLevelTo:(float)newVolumeLevel
{
    Class avSystemControllerClass = NSClassFromString(@"AVSystemController");
    id avSystemControllerInstance = [avSystemControllerClass performSelector:@selector(sharedAVSystemController)];

    NSString *soundCategory = @"Ringtone";

    NSInvocation *volumeInvocation = [NSInvocation invocationWithMethodSignature:
                                      [avSystemControllerClass instanceMethodSignatureForSelector:
                                       @selector(setVolumeTo:forCategory:)]];
    [volumeInvocation setTarget:avSystemControllerInstance];
    [volumeInvocation setSelector:@selector(setVolumeTo:forCategory:)];
    [volumeInvocation setArgument:&newVolumeLevel atIndex:2];
    [volumeInvocation setArgument:&soundCategory atIndex:3];
    [volumeInvocation invoke];
}