Now that I've wiped the egg off my face, I'm back to let you know you are correct.
Of the 24 mp3 files on my site, ony two had failed to upload. My first buyer happened to select one of those two to download, and I - defying all odds - chose the other one when running my test.
So I assumed there was a problem across the board when really only two files were affected.
Thanks for your responses.