Lotsa DVD's sound bad yes. It's however a combination of the datacompression and the audiocompression imo: DVD audio pumps a lot, I find that often very annoying. Like somebody lightning a cigarette in the movie pushing down the ambient rainsound.
I don't have a surround system, so i've been wondering if it could be cuz of some software algorithm that collapes 5.1 to 2.0. It seems tho that DVD's in general contain a seperately stereo audio track...
This stereo audio track can actually be an mp3, tho the quality of it can be set by the DVD encoder.
See here an overview of the possibilities of audio on DVD Video:
http://www.robertsdvd.com/video-audio.html
The only rule is the 6144kbps bandwidth limit.
So it really depends on how many audiotracks of what kind there are on the DVD. Several languages in audio (not just subtitles) eat up that bandwidth fast. And even if there are just a few audiotracks, they still might not use all bandwidth available.
Also, the better the surround audiotracks are, the fewer space there is left for the stereo.