There are no specific rule book references regarding crowd noise. Rule 9-9-1 is a “catch all” rule covering unfair acts by nonplayers and could be invoked by the Referee. Prior to throwing the flag, warn the head coach/game administrator of the team whose band is playing that further playing during a live ball could penalize the team. If the warning is ignored, I’d make it 5-yards for delay of game. If the playing persists, 15-yards for Unsportsmanlike Conduct. The generally accepted policy is that the band should stop playing when the offense breaks the huddle, or in the case of a no-huddle offense, when the snap is imminent.