I have same problem on brand new Slice - shows mute and volume level of zero. I've never connected a keyboard [it has no bluetooth adaptor either]
Also one of the leds is on all the time if that's related?
Discovered the mute/unmute option on the main menu 'Power' option.
A strange place to put that I would have thought! Would be far better to place this in one of the 'Sound' menus, or at least colour highlight it when muted.