"Unable to play file" (music) on my Samsung phone?

Hey guys,
I have been trying so hard for the longest time to put music on My Samsung Sprint phone...I know it can be done as someone once helped me put music on it via their computer...but now I am trying via my computer with a usb cord...(these are all songs from Cd's) and they seem to go into the media file ok, the titles show up; but then when i unplugged the USB from the phone and disconnected the phone from the PC, I try to play the songs and it says "Unable to play file." Can anybody help??? I would be forever grateful!!!

Make sure your file is either MP3 or AAC, as the M520 won't play other file types (such as wma).