You may want to change:
MPVolumeView *myVolumeView = [[MPVolumeView alloc] initWithFrame: _mpVolumeView.bounds];
[_mpVolumeView addSubview:myVolumeView];
towards:
self.myVolumeView = [[MPVolumeView alloc] initWithFrame:CGRectZero];
[self.myVolumeView sizeToFit];
[self.view addSubview:self.myVolumeView];