It might sound overly simple, but if you haven’t done so already; power off and power on again. Still no sound - check the speaker for debris - and if you have a small firm brush you can clean it out or even better is blue putty. If neither of these is the fix, check for updates in the system settings. Hope this helps.
Sounds like what’s sometimes called image persistence. Ideally you would start by powering the device off for an extended period, 24-48 hours, but assuming its your only phone and not practical to do so, you might try changing your wall paper to a solid color (white or black perhaps)and slide over to a screen with maybe one icon on it, or open an app that displays a solid color for awhile, this may help.
Like and comment below if you'd like to see more of a teardown? 👍🏻🛠️ p.s. anyone else seeing these newer Kindle's go into an unrecoverable reboot loop? I had hope the old 'disconnect the battery' trick would do it, but no. Seems more of a software issue and Amazon hasn't released anything to recover it, tho I'm fairly certain they have something back at their factory. Ours is now a Kindle Paperweight.