<p>A cappella, as a musical form, draws a lot of mockery from those who aren’t true believers. Its doom-doom-doom bass lines, all the splashes of lyrics from the backup singers, and especially vocal percussion strike outsiders as silly.</p><p>But what a cappella lovers love is the magic that the music can spring out of nothing but the gathered singers. It’s there, lying in wait, needing only the reedy tone of a pitch pipe to blossom.</p><p>The musical “Octet,” the brainchild of Dave Malloy (“Natasha, Pierre &amp; the Great Comet of 1812”), distinguishes itself from a 95-minute, no-intermission a cappella concert only by having the singers play characters, with some spoken dialogue.