Take a look at my thread. I'm not quite finished, but I'm getting close. Also, Ted White has a good article on the subject. If it's not on his website ( www.soundproofingcompany.com ), I think that he'd probably email it to you.
I didnt run HVAC through soffits, but built soffits over HVAC. Really not too difficult, just used 1/2 inch plywood and ripped some 2x4's in half. The only difficult part is if you can't find the ceiling joists to hang them from. Are your ceilings open still ?
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