I'm resurrecting this thread because I have this exact problem. I'm trying to play MP3 files served up via HFS.
I'm not sure what the problem is because I have a web host where if I have a direct link to an MP3 file, I can easily surf to the link in Safari on the iPhone and play the file by clicking on it. But with HFS, the iphone complains and stops with a "Cannot Play Movie" and "The file is not available." It's so strange because they both should be doing the same thing (easily playing the MP3 file), but on my host (apache) it works and on HFS it doesn't. And yes, I've added MIME type.
Any ideas?