Insulation, Radiant and Vapor Barriers - soundproof options

I have to help a friend with a problem: she lives in the first floor apartment of a two-unit (older) house and sound from the 2nd floor apt. travels through a heat vent in the upstairs kitchen right down through the duct, which runs down, exposed, through my friend's bedroom closet. My question is this; which type of insulation, of all the ones available on the market, is the absolute best as far as sound-deadening quality? Thanks, in advance for any input.

I don't know if there is any type of 'insulation' that can be used to muffle the sound and not diminish the quality of the duct work.. Or if this should be in this forum. However you may try a little bit of Dynamat inside of the vent, and the vibrations will quiet down a bit...

Dynamat is typically used in the car stereo industry, and blocks vibration. I'm not 100% sure this will work, but would be a good try for just a few $$$


-NFE

Thanks for the info., but maybe I wasn't as clear as I should've been. The sound that travels is voices, coughing, laughing, etc. Every little thing can be heard through the heat duct in the first floor bedroom closet, which is totally exposed/uninsulated. I want to wrap the duct with fiberglass, or enclose it somehow, and possibly even frame it in. Insulating it in some way can only help (as far as heat up to the second floor kitchen), I assume- I just don't know which type of insulation has the best sound-deadening qualities. Thanks again.