I did a search and someone said it was like blowing up a balloon is a box? Well is that why like in todays music you cant hear the instruments as good as older music. Like the cymbals or flute or piano sometimes. I use the srs plugin in WMP cause it brings out the quiet sounds. But is that what dynamic compression is? Cutting out the high and low sounds so the mids can be louder? I may have it all wrong. Also is there program that can undo the compression? Or emulate doing it?

Start with the FAQ, and go down to "Recording louder is not better," which points you to two links off-site.

Not much you can do to undo it; the best you can do is add ReplayGain (also in the FAQ) data to at least make the average loudness levels of different files match.

quickly explained dynamic compression is nothing else as: making the loud parts softer so that the softer parts get louder.

the compressor is analysing an audio-signal in realtime and adjusts the "volume" of the input regarding to the values of threshold (where it begins to compress), ratio (the strength of compression 1:2, 1:4 etc), and attack/release (how quickly the volume is adjusted). as the compressed signal is now softer than the orginal, you can make it louder with a gain-value (what also makes the noise louder)

compressing is done to get a "fuller" sound, and above all a "louder" one. the "louder" the recording "the better" ;-)

most house music is compressed to death, listen to some daft-punk tracks, and you will hear that the music is already "pumping".

compare recordings from the 70s with remastered versions of today, and you will hear that the modern recordings are much louder with reduced dynamics, because of compression.