Same problem here, but the default windows icon appears for mp3 files not only in LW, but in explorer too. I changed the association to musicmatch, the icon was good ! So i decided, winamp causes the trouble.
I used then the description in the previously proposed http://www.easydesksoftware.com/news/news12.htm page.
In regedit i looked after .mp3 key, found winamp.file in the first entry (default ?). Then I looked after winamp.file, found ....winamp.exe,1; changed 1 to -1 , as the description suggests - not good; then changed -1 to 2 : it's good !
(It's good for m4a files too.)
After every change I've rebuilt the icon cache by changing the display color depth in display properties.
I know, this is not exact, but works.