You absolutely need to account for sound while sneaking. Being behind someone doesn't require any skill but moving silently does, and should be greatly affected by what you're wearing. You should never be able to sneak up on anyone in a plate armor because every step makes a sound. But a disadvantage will have to do in 5e.
Then there's peripheral vision and head movement which means the "vision cone" should be a much wider 180 degrees. Right now you can basically sit in someone's field of vision without any cover but they don't even get to roll to see you.
