You can turn the ASL on or off by using the same knob you use to set the bass, treble, etc. Just keep cycling through all of the selections. My mom's RX400 and my dads new 4Runner has that feature and it is great. It adjusts the sound level according to your speed.
UoD, when I test drove a 2006 Limited with the JBL 6 disc radio, I think I remember seeing that feature when playing with the radio. I remember being impressed because I originally thought it was only available on radios that came in Lexus.