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Audio Primer
Sound in General	Sound is pressure waves that propagate through the air (or other medium - but not a vacuum!!) by slightly moving molecules in a towards-away motion. This air motion causes the same type of motion in our eardrums, which we perceive as sound. To accurately reproduce sound, the pressure waves must be accurately received, converted to something a bit easier to record than pressure waves, and put on a medium that has excellent retention properties so the recording can be played back at a later time with full fidelity. Playing sound from the recording involves reading the medium used in the original recording and converting the information back to pressure waves directed at our ears. If done perfectly, we won't be able to tell the difference between the original signal and the recorded signal. Since we have only two ears, it would seem logical that we only need to record two channels of sound - the signal that would originally have reached our two ears - to provide a perfect reproduction. The field of psycho-acoustics revolves around explaining why it isn't that simple, and even the best recording can still be identified as a recording. This field also attempts to explain the other end of the spectrum - why an absolutely horrid recording can still be accurately interpreted.
Analog Recording	Since audio is an analog signal (continuous - no discrete counting), our first attempts at recording were analog. The phonograph, reel-to-reel tape, cassette and 8-track are all analog devices. A variable amount of surface shape (for the phonograph) or magnetic flux (for tape) is a representation of the original signal. Unfortunately, all of these wear out with age, and the signal loses some of its fidelity for each generation of recording.
Digital Recording	We have very nearly mastered the art of putting a representation of an audio signal onto a medium that will remain accurate for many years. By converting the analog signal into a series of counts that represent the magnitude of the analog signal, we can use technology developed for computers to store the audio signal. The CD has virtually replaced all other mediums for storing audio, though there are those that argue that the fidelity is not as good as a clean analog recording.
Compression	Once we have a digital representation of an audio signal, we can make use of another computer- inspired technique - compression. Compression simply reduces the amount of data needed to store an audio signal with some loss in fidelity. There is a trade between compression and fidelity. Two popular compression techniques, especially when bandwith is limited, are MP3 and Real Audio.
MP3	MP3 compression is popular since it is tunable (frequency, bitrate and number of channels are selectable) and provides a good compression ratio. The most popular compression attributes for web applications is 44kHz bandwidth, 128 kilobits/second, two channels. This provides the full audio spectrum with near-CD quality in stereo with approximately a 10:1 compression ratio.
Real Audio	Real audio compression has been the answer to streaming audio (playing in real time, like a radio station) for quite a while. The sound quality hass improved considerably, with 56K modem speeds now able to play very respectable audio. The biggest advantage of Real Audio is that the beginning of a large file will start playing while the rest of the file is loading. The appearance is that the music is playing in real time - just don't try to jump ahead - the file will have to load up the that point before it will begin playing again.