This is a really good question. Based on my own experience, my answer would be "Yes and potentially no".
The truth is, animals communicate on different levels. Because we are all energy, and all our energy is connected, it's possible to communicate telepathically at a higher level than we are necessarily consciously aware. In other words, your horse can be munching grass in his field apparently oblivious to any communication.
However, having said that, I personally believe that animals are usually very aware that a communication is going on. Here are some thoughts to go along with this:
Some animals will appear to drift off or go to sleep during a communication.
Some will lie down quietly and not go to sleep, but look pensive or thoughtful.
Some animals will be alert, looking around as though wondering who's talking.
While preparing for an AIT session, an animal's 'voice' will often jump into my head saying, "Come on, can we get started now?" I am extremely aware of this when it happens, so I have no reason to doubt that animals are aware of me when I do the same to them (albeit more gently and diplomatically).
Animals can be listening even though they don't look like they are. Some prefer to look or turn slightly away to communicate.
Animals might also not be listening even when you think they are. If I try talking to my dog in the first 5 minutes of a walk, he's often much too preoccupied with the smells to want to talk. If you communicate and get nothing back, it's usually pretty clear.
If I don't feel I've got a good connection with an animal, I'll just stop and try again later. With therapy work, this is absolutely vital. Like me, animals have their good and bad moments, and I personally feel that expecting to communicate when they're in the middle of a Schutzhund trial, dreaming they're chasing a rabbit or doing the equivalent of the final furlong at Aintree is probably asking a bit much!
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