The reason the 1993 mix sounds good is because it is derived from the version that was used for 70mm prints, which is by far the best audio mix the film has ever had.
Unfortunately, they saw fit to paste additional sound effects on top of it when making the Definitive Collection laserdisc set. Some of these are beneficial; but others are loud, distracting, and unnecessary. The 70mm recreation (made by me) removes these additions, and is included in various fan-made preservation projects.
Just recently I did a direct comparison between the uncompressed PCM from the laserdisc and the 192 kbps AC3 found on the 2006 DVD. By importing both into Pro Tools and time-aligning them, I could seamlessly switch back and forth during playback by soloing one track or the other. I found that the lossy version holds up pretty well much of the time—during quieter passages the difference was subtle enough that it didn't stand out too much. But whenever there is music the clarity definitely suffers: there is a distinct loss of detail in the high frequencies, and the louder dynamic sections of the mix sometimes exhibit harsh digital compression artifacts.
Even the tape hiss is affected, for in the PCM it sounds smooth and pleasant, but after lossy compression it takes on a more harsh and garbled quality. Being essentially random high frequency noise, it is right in the range where lossy encoders start throwing away detail as the bitrate is lowered, and as an ever-present background to analogue recordings this can be a noticeable indicator that the fidelity has been compromised. Tape hiss may be considered the aural equivalent of film grain, which likewise is difficult for video compression to deal with accurately at lower bitrates.
I compared the two tracks by listening on hi-fi headphones, so the differences were quite apparent. With speakers the distance from the ear to the sound source is increased, so it may not be as evident, especially if they are of the type that most people tend to own. When encoding in Dolby Digital, 192 kbps for a stereo track is approximately equal to 448 for a 5.1 mix, and though far from perfect, this level is generally listenable on most material. Increasing to 320 kbps for stereo (640 for 5.1) is considerably more difficult to distinguish from PCM, since there are no overt problems with the sound quality at all, and any difference will be quite subtle. Nothing beats lossless for clarity, though, so you should always listen to that if it's available.