AudioServicesPlaySystemSound Volume?
I was also bit by a similar problem on this topic.
My problem was that there was a global setting that I was unaware of.
In the Settings app, under
General > Sounds > Ringer and Alerts
If 'Change with Buttons' is set to Off, then sounds using AudioServicesPlaySystemSound() will always be played at full volume (yet other sound API's such as AVAudioPlayer will respect the volume of the device).