I noticed the mushroom noise during play too...one team the Bruins played recently did this, so it was either New Jersey (probably not) or one of the three California teams. I vaguely remember reading something about continuous music being prohibited, but I'm not sure where I read that, and if it's an actual rule or a guideline. It's probably not a game issue - it's probably more of a copyright issue, because the regional sports networks may not have the right to air "Back in Black". You can play 4-5 seconds of it (like right before they drop the puck for a faceoff), but you couldn't play a verse and chorus.
Either way, the mushroom noise is much less distracting than, say, the booming voice that comes when they announce the scoring 1-2 minutes after a goal. I don't think it gives either team an advantage, since it's telling all players on the ice "Hey, the PP's done." Plus, I'm sure the players know how much time is on the PP clock anyway, and check it at every whistle.