S.H.
Get several fly traps.
It attracts the flies. They go into it. And can't get out and die in that solution in the bag.
Place it outside at key points.
Places like Home Depot will have it.
The sudden influx of flies, could be due to anything, or something at a neighbor's house/yard.
Or if there are animals around and a pile of poop somewhere, all those maggots and flies hatching from it.