iPhone keeps playing sound for few seconds after apps are fully closed

Somehow my phone will continue to play the sound of whatever I was last watching continuously until I turn my phone off and back on. It will last until I restart completely. I have tried everything to get this to stop all of the troubleshooting guides online and it still continues. How can I fix this?

[Re-Titled by Moderator]

Original Title: phone keeps playing a few seconds of a sound after apps are fully closed