DVDs use what's informally known as AC3 audio. Essentially some kind of Dolby encoding from Dolby Stereo to Dolby 5.1. Sometimes you find the odd DVD with DTS audio. You need a special codec to decode this audio which doesn't come with Windows. There may be others, but AC3Filter is one I remember using a long time ago.
You could also use all-in-one solutions like MPC-HC, VLC Player, and even XBMC. These can all decode Dolby and DTS audio without the need of any additional software. So if you're still not getting any audio using one of these, then there's something else wrong.
Hi i have a biostar tf570 sli a2+ motherboard with onboard sound by roland wavetable synthesizer a geforce nvidia 8600gt graphics card linked by internal spdif cable to the mobo out and and hdmi cable linked via a dvi adaptor the music in windows media center and media player plays ok but dvds dont HELP PLEASE THIS IS DRIVING ME NUTS o/s is windows vista home premium 32bit