I wouldn't do sand, as it can move/rearrange/leak. Try just taking some wood filler and applying it like spackle with a putty knife to fill in the low area before putting down the second layer of plywood. Once it dries, sand down any high spots.

weedyacres, great idea, thank you.

i plan to go under the house today (crawl space) to look from below. Shouldn't such a support beam go through floor and to a concrete footing? While maybe this one does, it doesn't look like it from above, that the post / beam sits on top of the plywood subfloor. Maybe it is / was ok to have it done this way so long as there is comparable support below in the crawl space. Interestingly, if that is the case (post sitting on the subbloor, supported from below), I see no toe nails to position it into place, so maybe it does go through. Just doesn't look like it. Odd...

Thanks again.