My guess would be that the player you are using, is not aware of the new files added. You should find a command there, somewhere, to refresh the listing of the files in your shared music folder.
I have 2 xp comps. networked together. The music in the shared folder worked well. Any new music we put in the folder cannot be acessed by the other comp. It says the file could be in use by another application but that is not the case. We restarted both computers and it does the same thing.

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