Your phone's microphone goes into speaker phone mode when external speakers are connected (i.e. your car stereo), and at the same time the audio is coming out of multiple, larger speakers whereas it would normally come out of one much smaller speaker. The software will still use the same noise cancellation algorithm regardless, so it probably can't filter out the stereo as well because it is tuned for the built-in speaker. Maybe there is an app for that?
Sent from my SPH-D700 using XDA App
"What is this?! When does this happen in the movie?!"
"Now, sir. You're looking at now - everything that's happening now, is happening now."