S video connector is used only for video, you need to connect the audio separately via the RCA audio connectors or via digital coaxial or optical connector.
You can read more on S video standard here:
Also see the related topics on audio, maybe you should consult the user manuals for your dvd unit and the amplifier that you use to get more info, but normally the sound can be connected via these connectors on the rear panel of dvd units:
- main left and right analog signal out RCA connectors (2 channel stereo signal - amplifier set to analog signal)
- main left and right, sub, center, surround left and right analog signal out connectors (multichannel surround signal - amplifier set to analog signal)
- coaxial digital signal out RCA connector (a single cable that takes the stereo or surround signal to the amplifier in a coded digital form - you need to set the amplifier to the digital signal mode to enable this kind of signal)
- optical digital signal out connector (same as the coaxial digital output, the difference is that the signal is in the form of light pulses instead of electrical pulses - the amplifier should be set to the digital signal mode as well)
Hope this helps somehow - if it doesn't, post me some feedback, perhaps a brief description of the units that you want to interconnect...