Upstairs was the teeming Cambridge Common; outside, the usual smokers' conversations. "Dude, you spit on my nachos." "No, I spit next to your nachos." And downstairs, Opera Boston Underground had returned to the Lizard Lounge for another well-attended performance. Proximity has a way of making up for bar-basement acoustics. Lacking any substantial musical education, this particular Bostonist has always enjoyed hearing such oversized sounds coming from human mouths, and this was the first time......