Have you tried playing the imported video in the Editing window? If you look at the properties of the video, you can adjust the audio volume. If it is clipping, you might be able to lower the volume here and help.
If that doesn't help, the only other thing I might suggest is to try to import the .vob files directly and see if it is any better. Don't use the import disc option, just open the DVD and look in the Video_TS folder and bring in at least one .vob file to test the audio.