You can of course demux the audio to a separate stream with VirtualDubMod & then if you author to a DVD add that stream back in. Not too sure about VCD/SVCD though.
Here's a link that might help for SVCD.
Personally I now use DVD2SVCD with the Cinema Craft Encoder but within the supplied software is a free encoder. Not used that but the rest of the pack is good. Don't be confused by the title , it'll do avi to VCD/SVCD/DVD as well as ripping DVDs. Google will find that easily for you.
It's just occured t me , it will also use TmpGenc as the encoder.