Usually a poor conversion. Some converters don't handle variable bitrate mp3 audio that is so prevalent these days. Which converter are you using?
You should always test your output before burning to a disk. If you need software to play dvd-video on the computer, try VLC media player. www.videolan.org
If you haven't already tried them, you should look at FAVC or AVStoDVD for free conversion programs and ConvertXtoDVD is a popular commercial product.