Most codecs do not have any configurable parameters; they are what they are, and that’s all they are.

Some codecs, however, are capable of behaving in different ways. This primarily means that they can be optimized for a particular goal, such as cutting down on latency, making best use of a network, or perhaps delivering high quality audio.

The codecs.conf file is fairly new in Asterisk, and as of this writing it allows configuration of Speex parameters only. The settings are self-explanatory, as long as you are familiar with the Speex protocol (see http://www.speex.org).

codecs.conf also allows you to configure Packet Loss Concealment (PLC). You need to define a [plc] section and indicate genericplc => true. This will cause Asterisk to attempt to interpolate any packets that are missed. (Enabling this functionality will incur a small performance penalty.)