Okay, I just tested the 6s and 7 in a quiet room. While playing back on the phones, you can hear loud hissing on the 7, and quiet hissing on the 6s if you turn the volume way up.
I imported the videos into Photos. Playing back on my Mac (connected to studio gear with Focal speakers), I hear almost no hissing in either clip.
My conclusion is that the speaker output on the phone is lifting high-end frequencies, and the iPhone 7 speakers are basically twice as loud. But the actual room noise picked up on the mics are basically the same on both phones. The iPhone 6s does seem to pick up more lower-range hiss; not sure why that is.
side note: I noticed that the iPhone 7 video (1080x1920) is recording at 30fps. Previously the iPhone always recorded at the film standard 29.98fps. Not sure why they changed that, but I'm not a film person.