This is because the 5.1 remix done for the special edition and the later blu-ray port for some reason mixes down the low end that was present on the laserdisc and VHS matrixed audio and the original flipper DVDs 5.1 track which merely was a discrete version of the earlier 2.0.

It is most apparent in the opening title after the quick fade just before the actual title comes up. When the Batman title appears there is a large amount of low end present on the old mix that is gone on the new one and really lessens the impact. It is unknown what made the difference, and I’ve often wondered if it’s perhaps the 35mm mix versus the 70 mm mix. However since the 70 mm mix did not have split stereo surrounds I don’t think the two were really all that different outside of the different formats.

Since the other three films also seem to have changed a very slight amount on their later releases I’m pretty sure WB did remixes from the ground up on all four films and conformed them to more modern specs in terms of EQ, dynamic range, and near field imaging. But it is the original film that really has the most audible difference when comparing the different copies. The rest seem OK but I swear by Forever’s LD matrixed audio which is better than the 5.1.