Advanced Linux Sound Architecture is an audio framework of drivers and plugins to manage the soundcard(s) on a Linux system.
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 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 by typing aplay -l into a terminal.
More information can be found at the ALSA project website