128 ks means that the audio uses 128 kilbits per second to generate the music, that's still pretty good considering the old audios which were 28 kbs, 44 kbs etc.
but 320 kbs is very high quality, the audio uses 320 kilobits of data per second to generate the song/music.. so of course the 320 kbs music has better quality
it's hard to distinguish the difference but we can tell the difference when there's a 'base' sound or drums sounds...