What are you talking about? AudioCD is encoded with 44100Hz sample rate and 16bits per sample, with two separate channels. It means it uses 44100*2*2=172Kbytes per second. It's far from different world than 128Kbit per second mp3 file. And you can easily hear the difference between those two formats. If you wan't good quality mp3 files, 256kbps bitrate is required (which can offer audible CD quality). You can also use some optimized codecs which can give audible CD quality with lower bitrates by using VBR (variable bit rate) mode, for example LAME with --r3mix preset. But never ever encode mp3s with 128kbps bitrate, it is not CD quality!