Sounds like the DD5.1 center channel is missing. This often happens when DD5.1 is improperly converted to stereo. It's probably getting lost either in the transcoding or playback. I would check the audio settings in both the transcoder and codec to make sure that any mixing or conversion is turned off and that S/PDIF and DD5.1 pass through are enabled. AC3Filter is pretty easy to setup for this. It's also quite likely that a different audio codec is being loaded for .m4v files (vs .ts) and it is improperly configured. That may require selection of a different audio codec in the media player.
If you are running windows I believe its called Gmplayer. Run the same command in a cmd window without using the GUI. If you want to post the output maybe I can help. This tool will show you if your recording process is down-coding from the original DD5.1 .
A utility called GPpot is also useful for identifying codecs required by files and what codecs are available and missing on the system. A program called GraphStudio is good for seeing multimedia graphs in real time but it doesn't appear to work on Win7, just XP. I find that nine times out of 10, the problem is with playback processing, usually a missing or misconfigured codec. The problem is finding out what is being used or is missing. Players like Mplayer and Media Player Classic Home Cinema have done a very good job of supplying and configuring codecs for playback. They are a good way of seeing if the file is Ok or for playback until a troublesome player can be fixed. Media Player Classic Home Cinema has a separate codec package available for download but the must be installed and configured manually. RadLight Filter Manager can help with that but it still tends to be a bit of a black art to most people.
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