It is amazing what you can learn if you read the manual. I've wondered for a while about speed sensitive volume on my 2000 LS. I've been through the menu a number of times to see if there was a way to turn it on. I was getting sick of having to turn up the volume as my foot became heavier. Long story short, today I decided to look in the manual. What do you know? There was the answer right there. If you don't know how it works RTFM!
