I copied some mp3s to my GT-N5110 ( wifi ) using Kies. Each song plays for only 2 seconds and skips to the next one. I have tried multiple 3rd party players and they each do the same.
I would try instead of Kies, just drag and drop files using the file explorer in, I am guessing you are using Windows. Delete the ones you moved with Kies first, so you aren't confused.