Its definately a fault on the remix. It was also there in the broadcast versions of the SE, as well as the NTSC and PAL laserdiscs too. I think Ben Burtt just got a little too bass happy and cranked it up too much so that it JUST distorted in a few places. Interestingly it's gone again in the 2004 DVDs if I remember correctly (one of the few things they DID get right!). I've only ever listened to all of them in two channels so as you suggested must be the main channels only? My personal preference for this is the 1985 LD release - the LF rumble is there but not in your face like the 1997 remix.