Install other codec pack wont help
i think the video he playback has multichannel audio track,
multichannel audio track wont automatically downmixing itself if the pc only have 2ch speaker, you have to dowmixing all 6channel into 2.
thats why the center channel aka voice is missing.

If you dont have ac3filter, download here: http://ac3filter.net/projects/ac3filter/releases

And read the guife here to do the downmixing yourself
http://ac3filter.net/guides/mixing_matrix