take a look at this thread here. close to the bottom the post by jahagan at comcast
I don't remember doing that manually but that is what mine are set at.MythTV sets the sampling frequency to 32kHz and the bit rate to something higher than the default. One or both of these will stop the audio.
Through the MythTV front end you need to adjust your liveTV and recording profiles. I set them all to 48kHz and a bitrate that translates to 10 (I think it was around 224kb/s).