One thing I have noticed is the difference between the audio volume level when you are moving as compared to when you are stopped. I am constantly turning the volume level down when I stop and then back up when the car gets moving; it is due to the road noise. I am wondering if anyone knows of a modification which would lower and raise the volume level depending on your speed. Most car have this feature but I can't find it on the Prius except for controlling the voice whenever the navigation system talks.
It was supposed to be there, but Toyota cut it at the last minute. Some have wondered if the programming is still there, but just missing the "swtich". In anycase I doub't you'll ever be able to add this feature. There are rumors going around that for the 2006 Prius the audio system may get an upgrade. If that is the case then perhaps you could swtich out your head unit with a newer one.
