You may need some way to average the response time of a circuit like this. You could wind up with every bump causing a thump in the speakers.

Yup, definitely has to be a part of the system. Not sure how to do that.

Not certain how long of a period it should be averaged for. Too long, and it won't respond to changes in road noise quickly enough. Too short, and the volume will vary too wildly.

But you're right, a simple op-amp circuit is the sort of thing I was looking for. I knew there was a simple way of doing it with analog circuits.