Have you got anywhere with this Tony? I find this a quite ingenious approach to the problem of road noise - for example, I did not think of 'conversatio feedback' problem with internally mounted mic.

To me as a total electronics layman, it woud seem to involve rectifying mic signal, amplifying it (with manual gain control), charging with it a capacitor in a simple RC circuit (that serves as an integrator - make R adjustable in order to control discharge time) and finally using the result to control the actual volume. Now, of course, someone who actually has a clue should step in.