Were you by any chance using .WMV files for your audio? I ask because the most recent occurrence of that error was when someone had a corrupt .wmv file -- or else Video Studio was having problems reading the audio files used in that particular project. Changing the audio to .WAV format made the error go away. Can't say if that helps in your particular case as I don't know anything about Video Studio's requirements for audio in a conversion to MPEG-4, or how much larger WAV files might affect the overall size of the rendered MPEG-4... The only other thing I can suggest is to remove your audio files one at a time and see how the rendering goes. That will require patience though -- but at least, once you identify the corrupt or unreadable audio file, you only have to convert one file to WAV.