This may be a bit too late in the life of this title for this, but for the sequel it would be nice to have some kind of headphone mode. Let me explain. As of now, the screen seems to be split in two: sounds either come out of the left ear/channel if the source is on the left side of the screen or the right ear/channel if its on the right side of the screen. Boolean style. There is a clear lack of "surround" to the various sounds and it just feels very unnatural. Headphone mode could blend the left and right channel with a function that keeps the sound perfectly even when on top of a sound source and blend it linearly by lowering the volume/amplitude in the opposite channel up to 50% (or what ever works) as you move the source to the other side of the screen.
An even preferable alternative would be to implement HRTF (Head Related Transfer Function) that works miracle for the 3D sound of games when using headphones.