Mp3. Not the best format.
First of all I am no expert in this subject, however this is what I know, Mp3ís are convenient, I use the format to send rough mixes to friends, but, and this is the thing itís a compression format.
Thereís something missing. It takes little bits of information away.
It is more noticeable at the high end of the frequency range, if itís a low quality encoding then you will notice it in the cymbal sound, there will be a noticeable discolouration in the sound.
In an ideal world you would use some sort of lossless compression like FLAC, but that makes for a much larger file size, as I said Mp3 is convenient and not the best way of transfering music.
It uses a psychoacoustic model to throw away the information must people will not notice being thrown away most of the time.
Take the 1812 overture.... when the canons fire.... if a penny whistle is playing you probably cannot normally hear it so you can throw it away, so you are not hearing the recording the way the musician intended you to hear it.
Itunes is not the answer to the CD. As now you see.
I would suggest you download a free media player that can handle all or most of the file formats that you can throw at it at Videolan.org