So is this now doable within GPSapp? Despite Tony's reservations about road noise I think speed controlled volume is a good idea. My girlfriend has this in her new mini and it works brilliantly.

Although the ideal would be to monitor noise levels this looks very easy to do with a blend of hijack, voladj and gpsapp. Presumably gpsapp has a real time view of vehicle speed - this could then be applied to the voladj algorithm?

The ambient noise version might be possible with a cheap external microphone on the vehicle (maybe a dry corner of the engine bay, but definitely away from the in car speakers to avoid strange feedback effects). A very rough and ready average volume indication performed on the empeg could then be used to drive voladj instead...