Under the "?" column, it shows an icon for the default player/application associated for that type of file. For example, if a search brings up an .mp3 file, and you have Windows Media Player set as the default player for .mp3's, the icon will be a WMP icon.
The blue music note is LW's default icon for audio files. For some reason it'll switch back to the default, though I haven't had this happen since many versions ago. It doesn't affect the files at all, they'll still play the same.
Try hitting the "Refresh" button on the library page, or shut LW completely down and restart it.
You could try deleting your LW preference folder and that might fix it, but doing so will reset all your option settings so make a note of them first. Instructions in this thread points 2 & 3(check both locations):
Fixes for Limewire.