Audio lost when converting Quicktime movie file to DVD file with WinAVI

I download movie trailers as QuickTime movie files, convert them to DVD(vob) files with WinAVI, and then burn them to DVD-R with Nero. About 50% of the time the audio is missing after I do the conversion with WinAVI. They download OK, I always play the downloaded Quicktime file first. After I convert them to vob I play the vob file with Nero Showtime player and often the audio is missing. I have Quicktime 6 and the latest WinAVI(7. something). Could it be a codec?

 

Some more info that may give a clue to what my problem is. If the Quicktime source file has audio when played with Windows Media player prior to conversion to vob, the converted vob file will have audio. If the Quictime source file does not have audio when played with Windows Player prior to conversion to vob, the converted vob doesn't have audio either. The source files always have audio when played with Quicktime prior to conversion. Any suggestions?