Here is an interesting demo I heard about on This Week in Media. It contains 2 audio clips in both 128 kbps and 256kbps mp3. One clip is Mozart and the other from R.E.M. The Mozart clip has a lot of strings in it. The R.E.M. clip has background vocals. Both are where you might find the differences represented. To be honest, I can't hear a clear difference. The strings feel a little more mushy but that could be psychoacoustic too. Certainly it's not the sort of thing I would notice without an A-B comparison. Give the demo a listen. Can you hear the differences?