Not possible change volume with + - buttons

[PetraSwed]

I have recently released an application for small children where I play short sound when they touch something. Users can adjust the sound volume as long as they have enabled "Change with buttons" in sound settings. But when they disable this feature than sound in my app is constant (which I would not think is so strange, BUT all the other applications react to the change of volume anyway. I dont understand what have I done to disable this feature. Or did I forget to do something to enable it? I am using this code to play the sound:


SystemSoundID pmph;
id sndpath = [[NSBundle mainBundle]
pathForResource:@"slurp"
ofType:@"wav"];
CFURLRef baseURL = (CFURLRef) [[NSURL alloc] initFileURLWithPath:sndpath];
AudioServicesCreateSystemSoundID (baseURL, &pmph);
AudioServicesPlaySystemSound(pmph);
CFRelease(baseURL);

any suggestions will be appreciated

Petra