Fixing/Converting/Down mixing 5.1 AC3 audio.

[Linux for blind general discussion]

Okay, so I have some AC3 audio in 5.1 surround sound that I extracted
from some .mkv files using mkvextract.

They play fine on my desktop using gst123, but when I attempt to play
them on my portable media player(a Blaze ET, which can play most audio
files I throw at it), I get what sounds like extremely slowed down
audio as if it's playing at 1/16 speed or something.

I'm not sure if the issue is some irregularity in the audio files, the
Blaze not knowing how to handle a 5.1 source, or the specific version
of AC3, though I'm leaning towards the second or third since some
stereo Ac3s I extracted from a different set of mkv files played fine
on my Blaze and gst123 needs the ugly set of gstreamer plugins to play
the problem files, but played the stereo AC3s just fine without the
ugly plugins.

So, anyone have any idea how to identify what's causing these files to
not play properly on my Blaze and how to fix it? I need a command line
solution.