So long story short I recently converted a basement into an apartment and plan on living down there. I insulated in between the joists and double drywalled everything and that is all ok. The only noise I notice is coming from the vents. At the time I didn't think to wrap the vents in insulation and I am kicking myself for it.
I've been reading about different duct liners you can get and I am hoping someone on here knows about one that can be installed in existing ductwork. I am currently putting filters on the vents to see if that makes a difference but it doesn't look like it so far.
Also if it makes a difference the voices seem to be vibrating a bit. I was considering using an acoustical caulk to seal where the vent meets the drywall.