I was getting a lift into work a couple of days ago in a colleague's Renault Laguna and I realised that his stock radio was turning the volume up and down as he varied his speed. As we accellerated up a local dual carriageway, the stereo's volume became noticably louder and then backed off again as we approached a roundabout. I thought that this was quite a cool idea.
Could this be done in Hijack by taking the speed signal from the ECU and connecting to the serial input? Assuming you know what wire you need of course. I know what it is in my car, I don't suppose it would be too difficult to find out for most vehicles.
Excuse me if I'm talking utter rubbish, I just thought it would be a neat feature