An exclamation point means the song file can't be located. It may have been moved or deleted.

The reason you can't play your mates songs is that the DRM stops you from doing so in some case's depending upon how the media is licenced

Strip the DRM from the media and you will be OK, I obviously do not endorse piracy and hope that you have an alternative legitimate source for this media such as a CD, how are these artists supposed to support a Lambo, Ferrari and Hummer if they do not receive the appropriate revenue

http://www.engadget.com/2006/09/01/m...r-for-windows/