ALSA is the Advanced Linux Sound Architecture, an audio framework of drivers and plugins to manage the soundcard(s) on a Linux system.
Authoritative information can be found at the ALSA project website
Configuration of ALSA is done system-wide with the file /etc/asound.conf, and for individual acocunts with the .asoundrc file. These files are documented in the ALSA wiki on the Asoundrc page
Diagnosing audio problems
If you aren't getting audio output from mythtv, you can try changing the device mythtv uses. In the third page of the General settings area, replace the word default in ALSA:default with a reference to your hardware device; for example, the second device on the first card would be ALSA:hw:0,1. You can find out more information about your audio devices with the command
$ aplay -l
More information can be found at the ALSA project website