Just came upon this. I'm sure you have solved the problem by now, but for others, it sounds like drain flies. They are just like fruit flies, but live in the drains. I had them a couple of years ago and tried everything. It was disgusting! I finally called my plumber and he told me to fill the sink with ice (lots) and then pour a lot of dishwasher detergent (the liquid) and a bit of water. Turn on the disposal and let it grind the ice mixed with the detergent. It creates a gross, black sludge and will come back up into the sink but it will go down. Just add it little water if it gets stuck. This did the trick for me. I do it now a few times a year to clean out the disposal and haven't had a problem since. If the flies are circling around the sinks they are not regular fruit flies.