I just took the shortest mp3 I had to hand: me saying the word "sun". Out of context it's not very coherent, is it? I've replaced it by a limerick.
If you replace it by your original mp3 file, it will play that.
I suggest you download the files on Gordon's site and compare them carefully with yours. There must be some little thing...
By the way, I completely agree with Stan's suggestions for breaking up your test into more manageable "bits". I always remember Martin Holmes' remark years ago: "The Internet is bite-sized".