You don't have to play a dummy sound. Using the AudioToolbox framework you can set the AudioSessionActive as follows:
AudioSessionInitialize (NULL, NULL, NULL, NULL);
UInt32 sessionCategory = kAudioSessionCategory_AmbientSound;
AudioSessionSetProperty (kAudioSessionProperty_AudioCategory, sizeof (sessionCategory), &sessionCategory);
AudioSessionSetActive (true);
This will allow the volume buttons to control the app volume.
See this question: Cannot Control Volume of AVAudioPlayer via Hardware Buttons when AudioSessionActive is NO for more information on this approach.