The problem may well be the format of the card. Did the computer require you do anything to the card prior to moving the files to it? If it was formatted by the computer, the phone may not read it correctly.
Also, did you just copy and paste, or did you make any changes to them? If you made changes to the files in any way it can cause this.
Another possibility would be that the program you are wanting to use them with will not read them from the card. Not all programs do. Have you checked the apps settings to make sure it allows for reading from the card? You might also try a different music player and see if the you get the same problem. If you don't, it is an issue with that app. If you do, then it is something with the card and or the files.