A long time ago I wrote about how to improve your iTunes song quality, but that article was about how to make the songs you already have in iTunes sound their best through a couple of tweaks.
This morning I was again listening to a song on YouTube, and then I checked it against the same song I have in iTunes, and the YouTube song quality sounded better than my song, which I purchased through Amazon.com. In short, that led me to find this amazon page where they discuss the (poor) quality of their MP3s, part of which is shown in this image. IMHO, I don’t think people are that concerned about 5 MB file sizes and 56k dialup connections in most places. (They could make this an option.)