Researchers have recently found out that human hearing beats the Fourier Uncertainty Principle, proving that the way music is compressed today leaves very much to be desired. Apparently the human ear has the ability hear the bits of sound the developers of compression codecs like mp3 had thought were safe to eliminate in order to save space.
If you're interested in good sound quality and the way you store the music that is precious to you, you should read the article linked below and understand that mp3 or any kind of lossy compression is not the way to go.
Human hearing beats sound’s uncertainty limit, makes MP3s sound worse