Dealing with HVAC vent air flow noise?

Photon46, I advise against opposed blade dampers located near the outlets. If you are going to install them, put them well upstream of the outlets, because the sound they regenerate has a chance to be reduced before it arrives at the outlet. I agree with Jeffreybehr's suggestion of larger flex ducts, but just make sure they are substantially larger than the existing metal ones, because the pressure drop in flex duct is an order of magnitude greater than in sheet metal duct, which your fan system probably won't be capable of handling which means not enough air will be supplied into the room. If you can only make the duct slightly larger than what exists, consider ducts made of fiberglass duct board which although they won't provide as much sound attenuation as flex ducts, the pressure loss isn't too much more than sheet metal. Slowing the air down is strongly advised, and oversize the outlets a bit. Using an opposed blade damper in the duct well upstream of the outlet instead of closing the damper on the register supplying into the room can help, too.

We're going to have the contractor start by making adjustments to the blower speed programming. While researching the issue, I found www.softairediffusers.com offers airflow diffusers that look to be of interest to anyone looking to tame noisy vents. They have engineering specs listed that show how effectively they reduce vent noise at a given cfm airflow.

Reducing the fan speed can help when the full capacity isn't needed, but the full airflow should be provided when the full capacity is needed and as such won't reduce the noise.

Essentialaudio, that's right. The blower is 17 speeds & programable with two differential stages. The plan is to reduce the blower speeds in the first differential stage when there's a 1 degrees variance from the programmed thermostat setting. When the second stage differential of 2 degrees variance happens, then the higher blower speeds will come into play.