As we grow up and live in this world we create all kinds of ideas about what the world actually is... we try to figure out what it all means, how we feel about it, what or who created it, etc. As we do this we don't tend to split the world into separate realms of sounds, images, smells, etc. On the contrary, we use all of these incoming sources of information to combine and integrate them with our pre-existing knowledge to form a more complete idea of what the world is.
For example, when we see a bird whistling we don't normally tend to think “Hey that's a nice sound! Hey that's a nice image! What a lucky treat that these two sensations which are in no way whatsoever related to eachother should coincide!”. Also quite unlikely would be the thought: "Hey, the way these visuals and this audio go together is really nice and well constructed!". More likely, we'll think “That's a nice bird which produces both this image and this sound.”... or actually, even MORE likely is that we'll simply think “Bird”.
We automatically think of the "Bird" as a whole, instead of seeing it in terms of distinct bird images and bird sounds. Well, it's exactly these “Birds” in our minds that I'm interested in. Both hearing and vision are entrances through which the outside world communicates to our conciousness what it's nature is. It's not just the individual entrances by themselves that are important to me, it's the message, the logic and the feelings that can be conveyed through their combined use. I create my own various types of “Birds”; my own systems in which sounds and images are inseparable, so people will view them as a whole. In doing so I try to create an alternate reality which resembles our own in its natural integration of all of the senses. The main advantage of this alternate reality is that I have much more power of it than over daily reality, which enables me to dictatorially impose my personal vision of beauty on it.