You shouldn't need ANY special codecs, other than DVD playback software, to play the DVD. It should install all that you need. I would say if you're using a fairly up to date PC that Divx/xvid would be all that you'd really need special (unless you're doing heavy, erm, internet video downloading
) but even then almost everything is moving to some MPEG-4 based format. It's not like years past when everyone had their own different favourite encoder. A few extras like Quicktime (or QT Alternative), etc... but none of that would affect DVD.
The fact that 2 different computers are both having trouble properly playing the audio has me pretty convinced it's just how the disc was made. I'm not sure of a 'free' tool to use to check the stream. TMPEGEnc/Tmpegauth (MPEG and DVD authoring tools) may let you check, check out videohelp.com for the links (not sure now).