I am using Amarok 1.3.  I had two songs in my playlist and I dragged one song above the other.  I clicked play and Amarok indicated that it was playing the song but no progress was being made, no song.  I played the next song okay.  I closed Amarok and opened, still this one song won't play.  I opened the song with Kaffeine and it played!

What on earth did I do?!

You could try upgrading to amarok 1.4 - so many bugs have been fixed that it's almost impossible to tell which one might possibly be causing your problem.