If you think that this is a new problem with the latest LW, you can go to this other forum & post there:
LimeWire Forums
Don't forget that each time you open LW, you'll connect to diffferent users for searching & so your results can vary a lot.
You could also try searching for
artist name - m4a, or
song title - m4a.
If you really don't want to see any mp3 results, you can add .mp3 to your keyword filter (tools > options > filters > keyword).