I was able to replicate your error. I previewed the page and kept that tab open in my FX browser. When I hit the preview button again I got the error message you posted. If I closed the tab I had open it would preview with no error.

Brilliant, Eric! I hadn't observed earlier if I had closed the browser window or not after previewing, but now that I do, I see it is consistent with your findings. And not only that, but if the preview window is not closed, a preview in any other browser will fail too. Once the page has been cached, it seems, I get the error if I try a new preview. And it seems to happen with the mp3 file too.

Thanks a lot, and also thanks to the other guys who tried to help!