One possibility for which type of fly they are is Cluster Flies, though we can't really tell without photos to examine, and this type of fly in particular is not a common nuisance in the summer months. Can you share photos of a few of the flies alongside a ruler? (If they don't hold still, try capturing some and putting them into the freezer to kill them intact; that way, they don't have to be swatted and potentially damaged in a way that will make identification more difficult.) You can also see if they seem to match up with the appearance of any of the fly types listed in our page Identifying Common Household Insect Pests.

Fly larvae require a source of moisture in which to breed, and this can take the shape of clogged drains, leaking pipes, wet organic matter (maybe in a crawl space, clogged roof gutter, or attic), and so forth. Fly adults, like many adult insects trapped inside, do tend to congregate near sources of light (lamps or windows), because they are trying to get back outside. Tears in window screening and gaps in door or window weather-stripping can be some ways that flies and other insects outdoors find their way inside.

Miri

Thanks so much for this information.  Currently, all flies are gone. Should they return, I'll follow up on some of these suggestions.

You're welcome, and we're glad to hear the issue has (at least temporarily) resolved itself.

Miri