I'd suggest you double check all your settings in TMPGenc, it could be a simple audio settings problem. I've been caught out a few times when exporting video and audio seperately, then burning the dvd and having no sound.
If its still happening, you could try converting the Quicktime files to AVI with River past Video Cleaner (
Download free trial here), then use the software you normally use with your burner, The basic Nero or Sonic package should be able to make an SVCD from an AVI file without too much bother.