I got a xiaomi redmi note 8. And today I experienced the same thing and it started when I was playing league of legends: wild rift and it becomes a hindrance in my game as it blocks the buttons I use to play the game. If you want a visualization; the volume always appear in the screen, every second so it doesn't go away. I tried restarting the device but the options to "airplane mode";"silent mode";"restart";"shutdown" don't appear. So, I long press and the device shuts down. Then when I try to open it go to Fast Boot. I tried doing what Mustafa said and in addition tried to clean the earphone slot with cotton buds with 99% alcohol. After that my phone goes back to normal. (I already have pried open the back of my device to try to fix the buttons itself but don't know how to get into the buttons itself.) Hope this helps the others before they pry open their device.