You don't mention the temperature where you live. Are flies still active outside? If so, have you checked
around the windows and doors to see that there are not cracks or holes that they are using to gain access from the outside?
Are there any garbage cans left close to an entry way or in the garage? Many people leave their garbage by an entry. Garbage attracts flies and when a door is opened they are attracted to the more favorable temperature inside. (air conditioning in summer, heat in winter)
It's best if you discover the source of the problem and fix it.
An indoor fly problem can com from a small animal carcass that may be in a wall or chimney. Your problem doesn't seem severe enough for this.
Fly traps and papers that you can buy at a supermarket or hardware store can be helpful for a less severe problem like yours. If it gets worse there are treatments that are available to professionals that aren't on the market to the public.