Have you noticed the second speaker grill on the bottom of the new iPhone 7 in place of the headphone jack? It's not actually a speaker, according to an iFixit teardown. Instead there's a plastic piece which serves as a barometric vent that lets the inside of the iPhone equalize air pressure with the outside. This is necessary because the phone is waterproof and sealed up now. That vent is critical because otherwise the altimeter wouldn't work.
Read more at 9to5Mac.com.