It could be one of two things:
1. The folder is no longer shared. You might check to see if the folder containing the mp3 files is shared in Windows. You might also right click the folder in the PC Share Manager interface, left click on "Property" and see if they're shared in the PC Manager interface also.
2. DRM (Digital Rights Management restrictions) may have been added to the files, which in some cases can restrict their ability to play. If you have since imported them to a music manager and that music manager added DRM, that may also cause an issue. The only way to determine that is to rip some fresh .mp3s (from Windows Media Player, for instance, in mp3 format) and see if sharing those files works.
Keep me posted!
I'm using PC Share with my LN52B750 and having trouble playing mp3. Just a month ago, the TV would find and play them with no problems. Now, all it says is "file format not supported" on all my mp3's. The exact same songs that played not more than a month ago are not recognized now.