What media player are you using to play the DVDs, is it Windows Media Player?
...If so, then download AC3 Filter from http://www.free-codecs.com/download/AC3_Filter.htm
It's a free filter which can decode PCM, AC3, DTS, MPEG Audio and PES (DVD), in multichannel formats. It also includes SPDIF support. It doesn't only work with Windows Media Player. It will will work with any media player which uses DirectSound.
There are also many features which allow you to manipulate multichannel audio, including matrix mixing, speaker output configuration and channel gain.