Okay, I think I figured out what the problem was. My AC3 encoder has adjustable settings for dealing with the high frequency cutoff point, and I seem to have inadvertently done this wrongly. The AC3 format's lossy compression throws away the most detail in the highs, I believe, and the range is dependent on the bitrate used, so that's probably why it was so noticeable in the 448 but the 640 came through mostly unscathed.

I've encoded the mix over again at both bitrates, this time with correct settings, and the sound quality is much better as it should be. I'll be uploading them shortly, and the links will be posted in my thread—I highly recommend that anyone who downloaded the faulty tracks delete them and replace them with the new ones once they're up. I'm not sure if the stereo and mono I provided were also affected, but just to be safe I'll do them over as well.

Again, really sorry about this error. I can't even begin to describe how embarrassed I was when I noticed it, but I guess that's what I get for not listening closely enough. My classes were starting up, and I was trying to finish everything before that happened, but . . . oh well. At least it was something relatively easy to fix, anyway.