What are you using to hear the sound? (TV,Optical,etc.)
Have you tried toggling your audio settings like the Dolby Plus and Navigation Sounds to see if that helps in any way?
If you can't get it to fix,try searching for an app that can change your volume settings.
Maybe a particular app is causing the stuck on mute issue.
Have you muted audio via within an app.
If you did,try using the same app to turn on the sound.
I also just read about Spotify causing a mute issue on Android,do you have Spotify?
This is all of the ideas I can think of right now.