I use Slack 10 since last week, but I'm using slack for about 6 years now... I've run into this very peculiar problem since upgrading, and I did a clean install from scratch (always do to eliminate old junk problems).

I can play MP3s from mplayer without problem, the sound comes in strong, and without touching the mixer values, which I monitor, I do

mplayer dvd://1 -xineramascreen 1 -fs -framedrop

without any options whatsoever, as I always have... But now, the sound is -extremely- muffled, and I have to crank the volume of the speakers to dangerously high levels which would be sufficient to cause a Back to the Future Feedback from Hell (TM).

My sound card is an es1370. Any ideas?

I assume you're using ALSA? How did you install mplayer? What version are you using and was it the same one as before? What about CDs, do they sound good or bad?

I use:
Slackware 10,
Kernel 2.6.7,
MPlayer 1.0pre4, built without any special options (./configure && make && make install),
ALSA which comes with Slack 10;

As I said, I can play with excellent sound mp3s and DivX movies, but -only- with DVDs the sound is muffled. Sound is OK in Windoze with WinDVD.

Is something required with newer ALSA to convert AC3 5.1 sound to standard stereo sound? I never had this problem before I upgraded to Slack 10... (The previous problem before upgrading was with video which now is resolved... I guess I can't have it both ways! lol)

Upgrade to 1.0pre5 and see what happens. Many bug fixes in pre5.

No change whatsoever... I'm really baffled, since it just happens specificaly with DVDs on MPlayer... It sounds like the mixer is almost at 0, yet /dev/mixer is linked to /dev/sound/mixer as it should, the 9-0 keys to adjust volume in MPlayer work fine with DivX and mp3, sound is lound and clear also in KDE... and this issue is present outside of KDE as well, so I'm really pushed into the corner for this one...

Maybe you need to upgrade your DivX/Xvid/MPEG-4 codecs. Check the Mplayer homepage at www.mplayerhq.hu for the latest.

Actually, after twiddling with the mixer controls, I figured what those weird mixer values -could- represent... Turning off everything with a -weird- name, like SigmaTel XXXX or IEC958 or AC97 2ch->4ch did some good... Apparently, with the new alsa drivers and/or KDE 3.2's kmix, those values are turned on by default... Turning them off removed the background issing along with giving at least a 6dB boost in volume, but the sound enveloppe is still flatter than in say, XMMS (with all equalizers off) or even MPlayer itself with DivX or MP3s...

When I mentionned that it did no good to upgrade to pre5, I did flush the codecs and reinstall the newer ones from this month... I always do a clean job! ;-) (If I could only do the same with my office... Then again, no vacuum cleaners comes with an "rm -R *" setting...)

I've noticed that DVDs tend to be recorded at a much lower gain setting than most other things. I don't know why that is. I too have to turn the volume up considerably to get any decent sound out of them but the sound is still very clear and noise-free. I'm afraid I can't think of anything else that might be causing your problems. I assume other linux DVD players work fine?