Exhibit turnovers are usually the least public, least artistic times of a gallery. At the hands of gallery director Pat Greene and the network of art-forward musicians in his orbit, however, the Gallery at Avalon Island now uses this brief window each month to present a relaxed but engrossing evening of live conceptual music that has so far included percussive, electronic, audiovisual and orchestral performances. It’s cultured, it’s progressive and it’s free (but donations gladly accepted).