Emily,
In most cases when wood shrinks around the nails you hear the floor squeak. Since you on the first floor, to eliminate that problem is very easy. If you have someone stand over the spot where the squeaking occurs, have them slightly bounce over that area making the floor to squeak, while the other person pin point the location from bellow. You will see the plywood move as they bounce over the floor joist. When locate the spot which will be next to the joist 90% of the time, there is a few ways to fix this...

One way, the most effective way, you will see the plywood movement up and down when someone is slightly bouncing in place and robbing against the nail. Take a shim or two if needed, apply carpenters glue over it and wedge it snug between the plywood and the joist to prevent movement and your problem is solved. If you have too add a few more in other locations. Make sure the shim fits snug and don't over drive it to lift the plywood.

Second way,drive a few screws from bellow up through the sub-floor into the hardwood near the joist where you see movement, but be certain that the screws is long enough to grab the hardwood, but short enough not to go through the face of the floor. I would suggest to have a contractor handle the second method if you not sure of the sub-floor or wood thickness.

Good luck