I've used the car stereo "auxiliary audio input jack" in my 2006 Civic LX sedan with an MP3 player, and with a satellite radio receiver. While I could hear the "speed-sensitive volume control" (SVC) working with the stock FM tuner in my car, it was sometimes difficult to tell if my car audio system was also automatically changing the volume of a device plugged into the auxiliary input jack (or "AUX" source).

To find out for sure, I turned my Sirius satellite radio on, and with its (low-level) audio output plugged into the AUX input jack, I set the car stereo system to a volume level of "8."

While going about 45 mph, I turned the SVC control from OFF, up to LOW, and thought I heard a change upwards in the satellite radio volume. Then, I turned SVC to MID where I clearly heard the volume go up. Finally, I turned SVC to its HIGH setting, where the volume increased dramatically.

Therefore, if the SVC did not effect auxiliary audio input at all, there should have been zero change in volume while adjusting the SVC settings. But I did hear the changes, so I now know for certain that the auxiliary input jack is affected by SVC.

Just thought I'd toss this out there, for those who may find such trivia of interest.