What the manual says is not wat u want to do. Its the midi_vol that do not allow the volume adjustment. And midi_vol variable affects the mixer(system) volume control. If u using master_vol that also affects the mixer and put off all sounds if set to 0. So this wont be appropriate for a game unless u want to mute off all sounds.

Note: The code above is tested and it works. In this way it affects only tat sound referred to by the media handle.