Silence (or at least the natural noise of the crowd and the game, with nothing added).
It seems like every freaking moment that's not disturbed by a pitch or a ball in play needs to have some sort of musical or sound effect additive. Walk-up music; between-pitch music; music blaring before the games; videos of bloopers/great plays/Godknowswhat between innings; public service announcements; etc. And it's non-stop.
It really hit me when I caught a Padres game in San Diego a few years ago. We got there early and were sitting, eating our hot dogs, and taking in the scene, which was a pleasant evening, but as time went on I was getting more and more irritable. I finally realized why - there was non-stop blaring music the entire time, not just playing but loud. It's not just Petco Park either - Dodgers Stadium has become more and more this way too, first under the Fox ownership and now even moreso under the McCourts, and it's particularly shocking to someone who grew up with a stadium without strobe lights and anything more than organ music in the background.
It's essentially entertainment for people with ADHD.