Thank you for your response.
The DVDs are burned DVDs but they have worked in the past. We had to re-build our operating system and after we re-installed Vegas, this problem pops up. The strange part is that when I play the imported files in windows media player, the audio works correctly. However, when I import them into Vegas, the audio does not appear. The DVDs shouldn’t have 4 channel audio because they were made from 2-channel media.