Animal movement sounds seem absent at times. Is this a bug or a feature?

[warmer]

Is it me or are some animals 100% silent at times while moving. I had a zombie bear go from 25 yards away to attacking me without ever making any sound. I was in the wasteland biome.

 

I was in my inventory doing something while sneaking and then the next thing I know I am getting attacked by a bear and a vulture. I heard the vulture flap it's wings and I exit my inventory and turn around expecting to see a vulture, but it was a zombie bear right in my face with the vulture behind him.

Further more, sneaking over piles of broken bricks should be super hard even for players. How is a zombie bear accomplishing this?