We've suffered from permissions problems with sharing Windows 7 music, pictures, and videos on the PS3 via DLNA. I believe the problem, in our case, was due to our having copied an old Music folder from a Windows XP machine. The fix goes something like this:
- In Windows Explorer, go to %userprofile%
- Right-click the folder for the type of media that isn't working (e.g. Music)
- Go to Properties and switch to the Security tab
- Check to see whether WMPNetworkSvc is in the list with Read permission. If not...
- Click Edit...
- Click Add...
- In the large text box, enter "NT SERVICE\WMPNetworkSvc" (without the quotes) and click OK
- In the list of checkboxes below, check the Read checkbox and click OK to close all dialogs
- In the start menu, run "services.msc"
- Right-click the Windows Media Player Network Sharing Service and select Restart
- Now select your media PC on the PS3. Perhaps now you can play your music, etc.?
Good luck! If you had to take any other steps to get sharing working on your machine, feel free to describe them in the comments.