You'll have to downsample the sound. In 5.1 dolby, your looking anywhere from 350 to 1GB for just the audio alone in a 100 minute movie. How's that gonna fit in there with your video as well?
Sorry, but it has to be done. The standard is MPEG audio anyway. You can convert it to a pseudo AC3 2CH and keep the bit rate up there, but it still eats up a good bit of bandwidth. The best bet for going to VCD/SVCD is to run it down to 16bit/44K MPEG audio or hi bit rate MP3 if doing a DivX.
You're gonna have to take a bit of a hit in audio to preserve video quality on a 700MB CD-R--but if you do it right, the casual listener may not notice.
If it ain't broke don't try to fix it! But, hell if you can tweak that bastard just a little bit more....hehehe.