I received great advice from here:
Soundproofing Products - Soundproof Walls, Ceilings, Floors and Rooms
This is Ted White's company. He is a member of AH forums, but I don't know how often he checks in. In the end, I didn't make a concerted effort to soundproof my basement, because I have an open stairway leading upstairs. According to Ted, if I couldn't seal that off, the time, effort and expense to soundproof the basement would have been of marginal value.
I recommend getting in contact with him.
