Has anyone had problems with the sound quality of downloads from iTunes?

I have noticed that the sound quality can be inconsistent. For example, I downloaded The White Stripes "Get Behind Me Satan" and Ryan Adams "29" from iTunes. The sound is not as crisp and and the volume is muted compared to other albums from the same artists that I have downloaded from iTunes.

Is this a problem with the engineering of these particular albums or a problem with iTunes? If it is a problem with iTunes, does anyone know if iTunes would allow me to download these albums again?

I had trouble with "Apologies to the Queen Mary," by Wolf Parade. One song stopped with thirty seconds to go, and another has an extraneous blip. I don't know if it was a problem with the download or with iTunes. But it's kind of weird. (I've even learned to love the extraneous noise in that one song !)