One Possible Fix For Crappy Sound
I was having a terrible time with sound on this D600 laptop; all my MP3s sounded like crap, with a distinct diminishing echo, emphasis on bass, and diminished upper bands.
Turns out that this D600 has a 3D sound card, and alsa had the "3D Depth" attribute right up. Running
alsamixer and setting the 3D settings to zero corrected the problem. Happy!