You should try "CustomVolumeStep" - it`s free and works with buttons.There seems to be 16 steps from 100% to 0% which isn't very fine control.
I use the "Volume Mixer" tweak on my jailbroken iPhone which gives me complete control over all separate volumes but doesn't change the step control for the buttons.
You should try "CustomVolumeStep" - it`s free and works with buttons.
But how to be without JB?