If this were my lawn, I wouldn't really worry too much about these weeds. In fact, if it were me, I would probably avoid the potential stress of herbicidal treatments given that the sod was only laid in December, although I realize that the Speedzone label seems to say it's alright. In any case, more frequent, low mowing will remove the bad appearance of the weeds, and may cause them to die out on their own. Start to ramp up your nitrogen feedings to get your turf more activated so the roots have enough energy to start expanding down into the soil.