I recently have been trying to configure a MythTV box for my computer (mythtv 0.20 on Ubuntu edgy). For some reason when recording, mythtv will record video but without sound. The solution is to unmute the audio for the capture device. See section 7.2 for the solution.

    $ amixer set Master,0 100%,100% unmute
    $ amixer set PCM,0 100%,100% unmute
    $ amixer set Line,0 75%,75% mute captur
    $ amixer set Capture,0 100%,100% captur
    $ su
    # alsactl store
    # exit
    $ 

Reference

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