There are so many ways to compress and digitize audio to be sent through the wire. A lot of codecs are available for use with FreeSWITCH, from ultra-wide band high definition (the quality of an audio CD) to the ultra-low bandwidth utilization, and all the variables involved can be confusing.
So, let's start with a bold simplifying assumption (we'll see complexity later): You only need to be aware of two codecs — G711 (which is available in two flavors: Ulaw and Alaw, also known as PCMU and PCMA) and G729.
G711 is the original, uncompressed format used since the beginning of time by telecom companies worldwide. It was designed to carry speech so that it only gets a very narrow audio band (300-3400 Hz), and to cut out the rest (humans can hear from 20 to 20,000 Hz; that's why music on hold sounds so bad on the phone). It samples that narrow speech band 8,000 times per second (8 khz sampling) in a logarithmic way (mimicking human hearing for different...