ALSA is the Advanced Linux Sound Architecture, an audio framework of drivers and plugins to manage the soundcard(s) on a Linux system. ALSA provides for automatic configuration of sound-card hardware and graceful handling of multiple sound device. As of the Linux 2.6 kernel, ALSA replaced OSS 3.x (Open Sound System) as the primary means for dealing with sound, though a backward-compatibility layer to OSS 3.x is provided.
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 General settings area, select your device from the list shown.