Software :: Mplayer/oggenc From Dvd: Why Does It Produce Output With Varying Volume Levels

I noticed that one DVD I encoded to ogg (vorbis) resulted in files with varying volume levels (some files have sound much softer than others... and the original DVD is not like that). I wonder if it's mplayer or oggenc's fault. I do it like this (suppose):

Code:

for i in 01 02 03 04 05 ; do mplayer-vc null -vo null -ao pcm:filename=$i.wav dvd:// -title $i-$i; oggenc $i.wav $i.ogg; rm $i.wav; done
I'm doing it by memory so I could have a mistkate or two in the sequence of commands but the idea is very simple

- Output the sound to a wav file using mplayer
- Encode it with oggenc
- Delete the wav file.

Why is the result having varying volume levels?