[mythtv] Prefer lower-numbered audio stream

[Daniel Thor Kristjansson]

On Wed, 29 Dec 2004, Doug Larrick wrote:
]I did some digging around last night, and here's what I found out.  AC3 audio
]streams (used by ATSC and DVD) contain information in their headers about
]libavformat/libavcodec expose none of this except number of channels, and even
]that is not properly initialized (always 2) by the time we use it in
]avformatdecoder.cpp.  My immediate problem would be solved if I fixed this bug,
]because it turns out the DVS stream is mono.  But IMO in order to make an
]intelligent auto-selection, having all the info outlined above would be
]helpful.
]
]Daniel had discussed some of this here before in the context of *recording* the
]proper audio stream:
]http://mythtv.org/pipermail/mythtv-dev/2004-March/020139.html

Most of audio information with ATSC is not in the stream that ffmpeg 
sees. Even the limited audio descriptor sometimes present in the PMT is 
jettisoned with the PMT rewriting. I don't think ffmpeg even tries to 
look at this information, but we don't save it in the recording to 
begin with.

-- Daniel