sound compressed sometimes

[xiaoz]

i always use vob2audio to get mp3 files, but sometimes i found the sound of the converted mp3 got compressed at some points, sounds as if the sound goes thru a compressor, anyone knows why's that? cos most of the time, my mp3 is fine, but some movies will have that effect, i don't remember i changed my vob2audio setup, it's always the same. vob2audio using wav same effect, but if i use dvd2avi, no compression.

Help.:confused: