before installing codecs you don't need, you may want to check the codec used for this audio using either
Avicodec or
Gspot
divx 3.11 won't interfere with divx5
I have a audio-only installer for divx3.11 if you prefer (can't remember where I got it, but I can email it or upload it somewhere).
there is an AC3 filter
here
otherwise the one I think Manao talked about is found on doom9.net-->downloads-->filters-->audiofilters.zip (direct link is not possible due to an anti-leech script on doom9's pages)