I bought mine individually carded. I didn't know the box set had sound? Hope you can repair it
Weird similar story, my son has a plush toy character Baymax from big hero six. When you press his heart he'd say phrases from the movie.
Now this feature stopped working a long time ago and I tried replacing the battery similar to the Hot Toys batteries but didn't work, so sealed him back up... Months later eerily Baymax started making electronic screech sound on his own, so I tampered some more and again couldn't get him to work. So I pulled the wires from the activation switch to stop the mentioned noise. Baymax is still loved and has his place in my sons room, but even spookier with all the connections removed only last week Baymax started speaking his phrases clear as day when my son gently squashed him
honestly it's a little creepy but it made us all do happy he worked again and still does