Hearing is Selective
A study published in Psychological Science in August of 2013 documented that we focus our hearing on familiar voices over those of strangers. We also can ignore familiar voices better...just ask my wife, Vicki! Sometimes we can hear but we don't listen.
As we age hearing becomes more of a challenge and in people with Alzheimer's disease a familiar voice can be very comforting. On the other hand, when we're especially focused on something (in the "zone") we're apt to block out everything else.