Sunken areas in lawn

Would there have been bushes or anything buried there after construction and these are now decaying and creating a void into which the soil overhead is collapsing into?

Maybe less likely but do you live in a limestone/karst area where there might be underground water channels in the rock that could be eroding away?

As Jim said, just fill the hollows in with topsoil and reseed

I'm not sure how the sand would help in hollows as big as those. You might consider taking the sod off one of the areas and investigating a bit so see what is happening. If it is something like tree roots that have rotted away then filling with stones and sub soil might be the way to go, then several inches of top soil. If you take off the sods you could replace them on top of the in-fill, probably add a bit more topsoil and re-seed..

dig out a big hole or trenches

line it with weed control fabric

fill it with small stones(pebble), use some drainage pipe also if you can

cover the stones with the fabric

stick about a foot of topsoil across the top... it will probably sink a little as it settles over a few months, but not much. lash a bit of sand over and level the gaps off next year

option