I just received the same error. I successfully burned a DVD (iDVD 7) complete with titles and chapters of home movies transferred from analog into iMovie 6.0.4. Then I decided to go back to my original iDVD file and add some audio to the menu section by going to the Media tab and dragging and dropping a song from my iTunes library. Tested it out in iDVD and it looks and sounds great but everytime I try to burn a DVD I keep getting this "There was an error during muxing preparation (project conversion)" error. It was working fine without the audio track, so something must be going on there. I've got 215 GB of HD space available.