I have Windows 7 (x64) build 7000 (the original beta build) installed on two of my machines, and I'm seeing a consistent problem with both of them and MC13.
If I reboot my machine and then open MC, ALL of my media files which live on network drives are all shown with Xs (missing) and will not play (gives the "none of the files were able to play" error). This happens even if I've opened the drive up in Windows Explorer and browsed around before I launch MC. If, after I launch MC and all the Xs are shown, I open the Network Drive up in Windows Explorer, it works fine. I'm able to browse around and play files from Windows Explorer using MPC and VLC and other players. MC won't play anything from my library though. It is like it just refuses to try.
To fix it, I have to close MC and then re-open it. Assuming I don't get the JRTools.dll error (which I reported in the current build thread), then it works and all the files play.
This happens EVERY TIME I reboot. I haven't really tested "timing" (to see if rebooting and then waiting a long, long time before opening MC makes any difference), but just using it normally I've consistently seen this issue.