Thought it'd be a fun challenge to play around with this, so have a look at what I've come up with:
Example capx. Drag and drop the green circle.
It's in two parts: setting up positional audio to get the left/right channel stuff working, and I used the distance between the sprite and the audio generators to determine their volume.
I tested this at quite low volume. Obviously it'll need tweaking according to your setup, so please start with the volume down low.
I do agree though, an attenuation percentage setting would be brilliant.