Developing for the iPhone, we wanted to show a volume control that sync’ed with the user’s volume control buttons on the side of the phone.  It turns out, the best way to do this is to show the volume control using the MPVolumeView class (part of the MediaPlayer framework).

Here is some example code from our app:

MPVolumeView *volumeView = [[[MPVolumeView alloc] initWithFrame:
                                CGRectMake(0, 0, 215, 22)] autorelease];
volumeView.center = CGPointMake(150,375);
[volumeView sizeToFit];
[self.view addSubview:volumeView];