Regarding Bf 109G-4, as explained by Thor, it is 99% the same plane as G-2. We could add it, but at the moment it is not on our priority list. You can substitute G-2 for G-4 without most players not even noticing the difference. But I doubt you can substitute a G-2 for any biplane or other plane that will come in 4.09 or next patches.
Changing sounds is a sensitive issue which is being discussed privately within our team. Nice sounds are not always the realistic sounds. We do have a member in our team who has first hand experience with WW2 plane sounds from the cockpit.