It was one of the site upgrades from about 10 days ago. You have to have imgflip pro, there is a new dropdown item for sound in the "more options" menu. I've made a couple, not sure if I'm going to renew my subscription though. It would be awesome if it also worked on click to show links, I asked Dylan about that. Give it a try if you still have pro, be sure to pick the "Enabled and Autoplay" button.
