Raise your hand if you wanted something like this. Don't be shy, Facebook claims this change was based on positive feedback...so it must be your fault. Thankfully, all you need to do is go in to your settings and turn off this feature.
Videos in News Feed have previously played silently — you tap on a video to hear sound. As people watch more video on phones, they’ve come to expect sound when the volume on their device is turned on. After testing sound on in News Feed and hearing positive feedback, we’re slowly bringing it to more people. With this update, sound fades in and out as you scroll through videos in News Feed, bringing those videos to life.