Does anyone know what controls the automatic volume increasing feature
There is "class 2 serial" data being communicated between the body control module computer and the radio. This is how the radio knows how fast you're going, it basically taps into the same data stream as the speedometer.
It increases the volume in discrete steps, at 35, 50, and 65 mph, if I recall correctly. How much it increases the volume at each of these steps is a function of the "SCV" (Speed Controlled Volume) setting, which is controlled by the ring around the volume control. Turn it all the way down (counter clockwise) and it is off: there will be no speed related increase. Then there are several steps, with the maximum increase being all the way clockwise.
The best way to test it is to set SCV to the max (clockwise) and accelerate to highway speeds. Then with the volume turned up to where you can hear it clearly, but not blasting loud, quickly slow down (make sure there is nobody behind you!) You should hear a distinct decrease in volume.
It is not as easy to hear the change when accelerating, as the engine noise tends to overpower the volume increase.
If it seems like the SCV is truly not working, it could be a loose connection. It will probably be a pain to diagnose.
Hope this helps.