I did some googling, I like the closed cell foam a lot, but it is
about 2 lbs/ sq ft. That's going to add an additional 600 lbs to my
ceiling if I pack the joist cavities completely. If I still go with
the double drywall, the ceiling will now be supporting about 1300
lbs. I see the OPEN cell foam is more like .5 lbs/sq ft, and they
claim it is also very good at absorbing sound. That would only be
about 150 lbs vs 600 for the closed cell..

I'm also finding things like rockwool used for sound absorption, and
rigid fiberglass panels like Owens Corning 703/705. I'll definately
be looking into resilient channels too. Maybe 1 layer of drywall on
resilient channels, that would get rid of something like 300-400 lbs.
Of course, if it isn't an issue at all, the extra drywall would help
soak up some extra sound..

The joist cavities are about a foot deep, and I'd like to pack as much
sound absorption as possible into there. How much weight can be
hanged from I-joists? Are my concerns about weight totally invalid?
I know ppl drywall ceilings all the time, but thats just ONE layer of
drywall, a few hundred pounds, vs the foam/insulation and double
drywall that will be about 1000 lbs more than a typical ceiling.