Fixes three regressions introduced in 3.51:

• Fixed that if you set an audio slider to 0% exactly, it would reset to 100% the next time you load the game.
  
• Fixed that standard and greater cover didn't apply circumstance penalties to ranged attacks against the creature in cover.
  
• Fixed that holding Tab would show everyone who had at least standard cover from the source tile as being in fully blocked line of sight.
  

I'm sorry for the regressions, and thank you for reporting them!