The transparency threshold of MP3 can be estimated to be at about 128 kbit/s with good encoders on typical music as evidenced by its strong performance in the above test, however some particularly difficult material, or music encoded for the use of people with more sensitive hearing can require 192 kbit/s or higher. As with all lossy formats, some samples cannot be encoded to be transparent for all users.

http://en.wikipedia.org/wiki/Mp3#Audio_quality

C'est possible d'entendre la différence entre un MP3 128k et un original, mais franchement, tout le monde est loin de pouvoir y arriver. Et sans tests en double aveugle, vous ne pouvez rien conclure. Sérieusement.