Audio is a huge part of any production and can make or break the viewing experience. There isn't typically a lot of preprocessing that needs to be done with well-produced audio, however. Raw captured audio may need some preprocessing (known as sweetening) to clean it up, but this is typically done during the edit process, rather than during preprocessing for compression.
Producing professional audio is its own specialized art, and those needing help with it should seek additional references. To learn how to get the most out of your audio production, try Peter Kirn's excellent book, Real World Digital Audio.
Volume is one of the elements easily adjusted in preprocessing, and you can do so in a few different ways. The quickest way is to raise or lower the volume, either by a decibel (dB) or percentage amount. This is another fairly coarse adjustment, akin to turning up or down the volume on your radio. For those needing a little more finesse, there are other possibilities, such as normalization and compression.
Normalization is the act of adjusting the audio levels in the content and then raising or lowering the volume of the entire clip so that the loudest sound matches the level you have specified. This is a global adjustment, affecting the volume of the entire track the same way, rather than affecting the relative levels.
This is a totally different type of compression than what we have discussed so far in this book; with regard to audio, compression refers to a specific type of audio filter known as a compressor. Loud noises in a digital audio track can cause distortion, and likewise, quiet sounds, such as whispering, can be lost. An audio compressor can smooth out these issues in an audio track by acting as a dynamic range. By pulling down large spikes and lifting those quiet parts up, compression will ensure that the average loudness is fairly constant.
Just as with video, there are also noise reduction filters for audio, although these are more often found in professional audio-editing tools, not in compression tools. Unwanted noise in audio tracks is just as bad for compression as noisy video—bits will be wasted, and the end result will be lower quality than desired. Some compression tools have simple hum-removal filters that will help clean up the audio during encoding, but truly bad audio may need to be preprocessed separately in a professional application such as Digidesign Pro Tools, Apple Soundtrack Pro, or Adobe Audition.