Ok, I'll never use Audacity to downmix again. It mixes the file way too loud, and creates clipping.
I've discovered a much, much better program. The program is called Eac3to and More GUI. Do a google search and check it out.
When you downmix a file, it detects if there will be clipping, and it lowers the mixing volume. Now Deadwing (which clipped quite a lot), now sounds absolutely perfect.
I tried to use the Downmix feature on DVD Audio Explorer, but it only works for a few DVD-A.s