http://en.wikipedia.org/wiki/Dynamic_range_compression
Compression can often improve the listening experience, but it almost always worsens the visualization experience. This is because it reduces the dynamic range of the audio, lessening the difference between loud things and quiet things. From a visual standpoint, it makes it much harder to determine the onset of drums and other percussive elements, especially when there are lots of other instruments playing at the same time.
Personally, when I want to create visuals for mixed music, I want as high of a dynamic range as possible so I can really see the pulsing of the drums and beats. Often this means I have to "un-compress" the audio (increase its dynamic range) to get the most pleasing visual result.
Fortunately, there is a quick way to do this in Magic that can make your visuals much more reactive. This is not the only way to do it, but it's probably the easiest.
Let's say you have a simple scene, such as the Demo Project that ships with Magic, where a Polygon's height is reacting to audio volume:
If you import your favorite artists' song into Magic and assign it to Input 0 in this project, you will probably notice that, when you play the song, the Polygon stays fairly big and doesn't really change size too much.
One simple way to make the Polygon's movement more dynamic is to add a Power modifier:
The Power modifier applies the mathematical function of x ^ y to the parameter. In the above example, I've specified a value of 2, so I'm modifying Volume by squaring it, or Volume ^ 2.
Try this and see how it looks. I think you'll agree it's an improvement.
Why does this work? Well, Magic measures Volume between 0 (complete silence) and 1 (maximum volume). In most mixed music, the loudest element will be the drum beat, so it will be closer to 1, and the other things will be somewhere below 1. Using our Power modifier, we can see that, if the input's volume is near 1, the modifier's output will also be near 1, because 1 ^ 2 = 1. However, if the input's volume is around .5, the modifier's output will be .25, because .5 ^ 2 = .25. In this way, things that are loud will stay loud, but things that are quieter will get even more quiet.
For an example of this in action, check out the DrippyStarfields sample project included with Magic. The Starfield's Size parameter has a Power modifier added. Try importing one of your favorite pop songs, and experiment with changing the Power's value.
This can all be a little bit technical, but it's well worth learning about, because it's a common issue especially for DJs who don't have access to the individual tracks of the song. If you're working with a live band though, and you do have access to the drums' separate microphone(s), you might not need to worry about this issue at all.
Please feel free to reply if you have any questions!
Eric