Say you have an audio AC3 file and you want to convert it to WAV in order to work with it in Audacity or some other audio manipulation application. One of the methods is to use mplayer, included in the Debian Multimedia repositories.
The command to convert the desired file to WAV is:
mplayer -ao pcm:file=output_file.wav input_file.ac3
This command will convert input_file.ac3 to output_file.wav, using the PCM audio output driver.