Try this:
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(volumeChanged:)
name:@"AVSystemController_SystemVolumeDidChangeNotification"
object:nil];
- (void)volumeChanged:(NSNotification *)notification{
NSDictionary*dict=notification.userInfo;
float newVolume =
[[dict objectForKey:@"AVSystemController_AudioVolumeNotificationParameter"]floatValue];
//my comment: newVolume is value that you need
}