Stumped...

Max got this book when he was probably 1 or 2, and for one reason or another, it accidentally ended up at the bottom of a box that was placed in his closet, before it was even read I think. I completely forgot about it until a few months ago when I came across it while cleaning out the box. I thought the boys would love it because it's Elmo and more importantly, it had a button to push. So I sat in the middle of the two and began to read the book.
After a few pages, you're instructed to push the button that plays Brahms lullaby. Carter immediately pushed the button and before I knew it, Max stuck his bottom lip out and began to cry the saddest cry I think I've ever heard from him. I comforted him and tried to figure out why he was crying because he was truly upset. I thought back and realized that the music had played right before he began crying. But, for heaven's sake, the Brahms lullaby couldn't possibly be what upset him... was it? So I decided to test it.
By this time, he was happy and I continued to read the book. Then it was time to push the button. I pushed it, and sure enough, Max started crying hysterically and it took a good couple minutes to calm him down.
What? I was so confused and quickly put the book away because he was tearing my heart out! The other day Carter found it and wanted to read it. Max immediately pushed the button and when the music played, the same thing happened.
What is happening? I could understand if it was an ear piercing heavy metal song, but a soothing lullaby? Is he relating that song to something traumatic that happened to him? I am seriously stumped. But I know one thing... I cannot stand to hear that cry again so I'm not sure I want to get to the root of that problem if it means playing it again.