Ron has a 40" Samsung TV with an Amazon Firestick. He can't raise the volume up or down. It's just stuck where it is. Leo says that it's likely an issue with the TV, not the Firestick. Leo says that he thinks that there's an issue with the TV's CEC option, which allows him to control the TV with the Firestick remote. What he suggests is to disconnect the Firestick and see if the volume issue persists. If it doesn't, then he knows there's an issue with the Firestick.
Leo also suggests reprogramming the remotes to operate the TV. ScooterX in the chatroom suggests looking in the TV menu for a reset option in the TV settings. That could bring it all back to the factory default settings. Then try reinstalling the Firestick there. He can also go to Samsung's website and look for a firmware update. Bring it up to date by installing the update. That would clean up all settings back to default.
Settings - General - Smart Features - Autoruns Smart Hub - DISABLE. Here's a link. https://cineplex.zendesk.com/hc/en-us/articles/115006205047-I-m-getting-low-volume-on-my-Samsung-TV-
Here's a discussion on this very problem with the answer - https://us.community.samsung.com/t5/HD-and-UHD-TVs/volume-gets-stuck-and-won-t-move-up-or-down/td-p/1523751