I downloaded some MP3 files from lala.com, and put them into my shared music folder on a Vista computer. Surprisingly, they didn't appear in my music player when I connected to the Vista machine with a client Windows XP machine. I could see all the other MP3 in the folder, but not the files from lala.com. I right-clicked on a working file and compared the file permissions to a non-working file. Sure enough, the non-working file was missing permissions for "authenticated users". I guess the lala app normally creates files with restrictive permissions. Windows explorer, when making a copy, will make files that are readable through Windows file sharing. I tried to fix the permissions through Vista's dialog boxes -- a hugely complicated and difficult process.
Instead, I found this:
icacls c:\ben\music /grant "Authenticated Users":F /t
This fixed all files in one command!
You can run icacls all by itself to see its usage.