knockon not working / sounds not playing

I have an LG G2 mini, and the turn on knockon feature never worked at all. "Settings/General/Gestures" show the option "grayed out", although activated. I can't deactivate it, and on top of it, as mentioned, it doesn't work (I have to press the button on the back of the phone to turn the screen on) :\

Well, I can live with that. But I also got used to the sound the phone plays (played) when I double tapped to turn off the screen (that little tv screen off sound, that always worked). But recently, it just turns off, and no sound is played at all.

Why all that happens? How can I fix that? Are those phone bugs, or are they "Android related"?