Nothing you put on the outside of the duct is going to help this, because the sound is traveling along the duct. The same as the speaking tubes they used to use on ships.

You may be able to cheat and absorb some sound with an air filter. Or if ducts are way oversized (as in plenum or joist chase returns) you could line them with acoustic board at openings. You may also be able to get noise blocking diffusers.

But mostly I think you're going to be stuck with this unless you remove and replace with duct board or similar.