If your drive says DVD, then you have DVD hardware. It's playing the DVD, so you know it works.
Usually you have more than one kind of DVD software on your computer. For example, Windows comes with a multimedia player, and the computer hardware often puts in DVD playing software, too. So try a different software.
Take the DVD out. Now put it back in. Does it just start playing, or does it offer you a choice of applications for playing the DVD? If it just starts playing, then someone set a default app for it. So shut the DVD off, go into your Start menu and try it with Windows Media Center or whatever.
If you're still getting picture but no sound, post back with what kind of computer you have, what version of Windows and what kind if DVD playing software you tried.