CoreAVC is a good H.264 video codec, I assume you are seeing the video just fine, it is the audio that is being recalcitrant.
What DirectShow AC-3 Audio decoder do you have installed?
If you open the .mkv in GraphEdit or GraphStudio what does the default graph look like? Does it play both video and audio correctly?
Maybe you could try FFmpegSource for the audio.
Code:
V=DirectShowSource("C:\Users\Floren\Videos\Sample-orig.mkv", fps=23.976, convertfps=true, Audio=False)
A=FFAudioSource("C:\Users\Floren\Videos\Sample-orig.mkv") # You may need some options
AudioDub(V, A)
Info()
ConvertAudioTo16bit()
ConvertToMono()