I think cars should some sort of soft horn that you can press to alert nearby pedestrians that you are there without the jarring sound of a full horn, and I don't see any reason not to make it customization.
I don't see any reason not to enable applications to see how far the pedal is pressed, assuming you give permission to the app to do so, and that could be used to make fake engine sounds come through your speakers.
But I for one don't want fake engine sounds.