I'm trying to diagnose an iPhone X which continues to play audio despite having turned the audio all the way down. The sound produced is nearly imperceptible but if you're in a silent room, it's still making audio. This is happening when playing web-based videos in Mobile Safari. This has also happened when playing videos embedded in Tweets displayed in Tweetbot. The audio control is swiped all the way down in the control center, and I've also pressed the left rocker all the way down during playback and the screen shows the "Volume Mute" overlay. It still makes sound. How is this possible?