No sound from behind

[Ultron]

Mantas are pretty darn stealthy....

[eXeCuTioNeR]

Hey guys!
Another little question..

Why can't I hear anything going on behind me?
Well, the speaker works fine, tested it and it plays several sounds during game, that's okay..
BUT the most critical thing is that I don't hear anything going on right behind me, like Mantas.. Or are they supposed to be "unhearable"?