Sub Floor Reinforcement
Although I'm unaware of your joists' specific spans, I don't think that's your biggest problem.
For a 19.2" (common spacing) on-center joist spacing, use of anything less than 3/4" subfloor is a mistake. So yes, adding another layer of plywood would be a good idea. In the old days, many people used a layer of 5/8" and a layer of 3/8", and it makes for a very solid floor.
These days, 5/8" plywood is for roofs and doesn't see much use on floors.
What type of flooring is on this floor?
You mention that it feels hollow. Can you elaborate? Do you feel the plywood deflecting between the joists under foot? Or is there noise besides the squeaking and popping?
The squeaking can be minimized by screwing the existing subfloor down to each joist every 8 or 10". I'd use 1-1/2" or 1-3/4" screws.