I've noticed the vent slats at the rear corners of the cargo area. I just thought they were speakers, but after looking at the service manual, I realized that except for the woofer, the speakers only go back as far as the second row doors.

I am wondering if this might be the source of all the road/rain noise?

What purpose do they serve? No they are not HVC either.

To vent the interior. This is espectially important when you close the doors as it relieves the air pressure. My Altima had a similar vent.

BTW, I feel that the source of much of the road noise has to do with the type of tires. Personally I do not like the Continentals mine came with. Even then, this is a hatchback vehicle and it will have a tendency to be a more noisy than a sedan.