Why do we turn down the car's music system when we're looking for a particular street address?

It falls under the classic definition of "sensory overload". The input from our eyes and ears travels to our brains as electrical impulses via a series of neurons. The impulses often share a common pathway up to a certain point, until they're "translated" and sent to the proper part of your brain. So while you're attempting to concentrate on reading house numbers and street signs, your brain is having trouble cooperating because it's also busy mentally singing along with the song. Even if you don't know the lyrics of the song that's playing, you're still being distracted. Studies have shown that any type of noise, whether it's a throbbing bass beat or someone in the passenger seat saying "I told you we should've turned left back there", increases the production of stress hormones. Stress has a negative effect on short-term memory. And if you're late for an appointment and lost, and are straining to read addresses in the dark, you've got more than enough stress on your plate without that background noise.