I too experienced the same problem.
If I play my original dvd (using POWERDVD XP) and then play my backup dvd+r, the audio volume is definitely LOWER on the backup dvd+r (same system , same setup, full dvd-backup). I also tried the same experiment on my standalone player (DAEWOO DVG 3000N) with the same results.
I understand what is being said about the audio not being 'touched' in any way whatsoever, but maybe some other factor is causing this problem.
There are obviously MAJOR DIFFERENCES between the backup dvd+r and the original dvd, such as disc reflectivity, encoding, format etc, etc ......maybe something in there is causing this anomaly ???
I leave it to the experts.
1.7 Ghz P4
Asus P4T motherboard
Sound Blaster Live 5.1
80 GB HD -Maxtor
Sony DRU500A burner (TDK 2.4 x +R Media) Firmware 2.0F
Toshiba SDM 1212 DVD ROM
WIN XP HOME
DVD DECRYPTER 3160
DVD2ONE ver 113