How to seal this crack in plywood subfloor?
Well it would help to have more information, This looks like cement backer that was cut. not a crack created by normal movement. If it is a crack RUN OUT OF THE COUNTRY! Because cracks don't show up 1/8" wide and and are gone in this short distance. The noise problem could be caused by the structure., not being glued and screwed or just cheap construction. I am not sure it really needs to be filled and I am sure it is not the root cause of the squeak. If you or any one out there can develop a "crack in plywood " anything like this you would make the record books. But if it is just old vinyl glue that makes it look like backer the problem may be more screws are needed, maybe some shims from the bottom. But filling the cut won't help.