According to my acoustics book, a couple of things to try are: lining the duct with some kind of sound absorbent, and/or placing a sound-absorbent baffle in front of the duct that still allows air to pass.
Lining an existing, installed duct could be difficult of course, but one possibility might be to spray the inside of the duct as far as possible with that sound absorbing spray the car audio guys use. If I understand correctly, just dampening the last couple of feet or so of a run can make a difference.
I've also heard of installing staggered baffles inside the duct but you have to be able to access it (I toss that out just in case it's practical to do so, or you are really feeling ambitious!)
For the external baffle, I think a piece of plywood or similar material, several inches larger than the outlet and spaced out from the outlet to allow air to pass around it might be effective. Acoustic foam or acoustic tile adhered to the ply would add absorption. Paint or wallpaper the outer surface to match or harmonize with your room (or wrap in fabric) so it isn't ugly.
So much for my amateur opinions. Maybe some pro installers will chime in here with more informed options!
