On my SE I use the OK button on the right side of the steering wheel to change to Line In when in Radio mode. Effectively muting since there is no input at the time. Of course if you are using Line In it will just change to whatever radio station you left it on but I don't use Line In that much so it works great 90% of the time. Just sayin'.
Candy Blue SE HB, 6sp Auto, Sport Package for the wife
Sterling Metallic SE HB, 5sp manual, Sport Package for me