this is because a high bitrate MP3 above 256kbps discards information that is physically inaudible by the human ear, information the brain will discard by itself, you will most certainly not hear a difference between 256kbps and a lossless file ever. no matter how good your ears are, it is just impossible. it's the same thing that goes for video compression and image compression, you can't tell a blu-ray disc is compressed can you? no....that is because the information discarded could be safely discarded without affecting what the viewer sees. the brain interpolates a large amount of the information it receives. the only time mp3 begins degrading the sound quality audibly is when people use lower bitrates, lower then 192kbps, then it is forced to throw out information you can hear, which does affect the sound quality (duh). there are also several much more effeceint codecs such as WMA and AAC that do a much better job of compression. both WMA and AAC at 128kbps cannot be distinguished by even a trained listener from the original, so using AAC or WMA at 320kbps is double that bitrate and you can put your mind at ease that nothing you can hear has been removed from the audio.