At 1-6, they’re having a less-than-successful preseason, but the Celtics are still doing their thing off the court. Celtics MarShon Brooks, Kris Humphries, DeShawn Sims, Kammron Taylor, and Chris Babb were at Mozart Elementary School on Monday, reading to kids as part of the Read to Achieve program. . . . And judging from the turnout at an open audition to sing the national anthem at upcoming Celtics game, there’s excitement about the team. More than 100 singers/musicians showed up, where they were judged by the likes of radio peronalities Braden Moriarty, Pete McKenzie, Gregg Daniels, Sue Brady, Karson Tager, and former “TV Diner” cohosts Jenny Johnson and Billy Costa.