hairy_hen said in this post:

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.

Which encoder are you using - Aften, or professional Dolby-certified software?

IIRC, the pro encoders I've used in the past (Surcode and Sonic Foundry/Sony) have just a low pass filter that can be turned on or off, the purpose being to roll off high-frequency content that could cause artefacts in the encode. It is recommended by Dolby to turn this filter on.

I think that the audio bandwidth is adjusted automatically by the encoder; at low bitrates the bandwidth is decreased in order to maintain high audio quality. However, I wouldn't have thought that 448kbps (for a 5.1 mix) would be problematic, and certainly not 640kbps.