Insulating and reducing boominess of plywood subfloor
Stuffing the open area with fiberglass will absolutely help, as it will dampen/absorb the sound waves resonating between the joists.  It will only go so far, though, as a significant amount of energy from foot fall traffic will still be transmitted through the floor and into the joists, causing the sound vibrations in the first place.

To combat that, you'll want to dampen the energy (convert it to heat) before it gets to the joists.

Step one is to add mass, if you are able to.  If you can raise the floor a bit, then add one or two more layers of some kind of dense sheathing.  Concrete board works great, but is expensive and hard to work with.  3/4" (19mm) OSB would work great.

Step two is to add a damping layer between the layers, to break up the resonant frequencies between the like layers.  At minimum, something like 30lb roofing felt would work.  A custom-created product like Green Glue would work even better.

Step three might be to add a final damping layer designed for foot fall traffic.  There are quite a few brand names out there and they all work roughly the same, by maintaining a densely flexible layer just before the subfloor.

This site has some really good info on this topic.  They are trying to sell you a product, but even if you don't go that route, the rest of the info is very solid: http://www.soundproofingcompany.com/soundproofing-solutions/soundproofing-floors/