hairy_hen said:
... Dolby Digital decoders when downmixing automatically engage non-defeatable dynamic range compression in accordance with the DRC profile specified in the AC3 bitstream, the end result of which is a hideous reduction in the power of the soundtrack.

In my experience, DRC is always defeatable - there has been an option to turn it off in every DVD player or Dolby Digital receiver I've owned.

For dialogue normalisation, however, the opposite is true.

(Edit - oh wait, you're talking about the downmixing, not decoding? I don't know then...)