The smorgasbord of sound known as MixFest got underway Friday with a special pre-party at the Revere Hotel featuring Matchbox Twenty boys Rob Thomas (left) and Kyle Cook. (We dropped by on the early side to see the duo’s sound check.) Saturday’s main event at the Hatch Shell includes performances by Train, Gavin DeGraw, and Andy Grammer. The fun begins at 4 p.m.