MP3 is a lossy audio codec. Even the highest bitrates, either fixed or variable, are still sacrificing audio information. With good encoders like LAME, this is not as much of an issue, but other encoders tend to be not so great and in my experience make the audio quite muddy and poor. Certain types of music also suffer more than others; in rock music that has distorted guitars and even overdrive on the whole mix, the distortion and artifacts the MP3 encoding causes will not be noticeable in most cases, but in classical music, pop music, softer rock-based music, etc., MP3 is not nearly as flattering.ChaoticLegion said:MP3s can sound just as good as a DVD copy, you just need to get them at a high bit-rate so as not to lose the quality.
Furthermore, you are still going to be limited by your equipment. If you are listening on anything but audiophile headphones, studio monitors or expensive tower speakers (in the $2,000 or above range), you likely will not notice a huge difference, especially if you don't have a good amp or preamp in use. If you have spent a good deal of money on a high quality stereo system of some sort (no, that stuff you bought from Best Buy does not count) then the difference will be immediately obvious to you unless you've got a set of tin ears.
The issue with MP3 is that while it provides "good enough" audio fidelity for most people, for those who want uncompressed, truly high-quality audio, it simply does not cut it (even CDs have their downsides compared to the original source material). The move towards MP3 because of its increased convenience and popularity means that we may see the death of high-quality audio in the consumer market, and it would be a complete tragedy.