[edit: well it was on at the end of page 1...]
in the end, it isn't too much about what we can hear rather than what it is whether we hear it or not. MP3 IS compressed, and vinyl sound or other analog recordings are NOT compressed, they emulate true sound. Though I know I can't tell the difference, it is just technically so.
Actually, in my Spanish class, we used to play a lot of Spanish songs, by Latin pop artists. They were all on these burned CDs, and most of them were God awful mp3s and I could hear all sorts of artifacts while listening to them (I actually pointed it out one day and realized I was the only one who cared). I don't know how old the CDs were, and therefore how old the codec was, but still.
Are .ac3 files 16 bit? Doesn't that mean it's just CD quality audio?