Safari 5 running in Snow Leopard has been a problem for me. It screws up html, loads a distorted control bar in HTML5 and crashes every now and again when I do a Google search. Reporting these issues is a waste of time as the latest update had no fix. I test all my pages in Firefox and Chrome and don't bother much with Safari since less than 3% use it.
I have the same issue as the OP. The file will only load in Safari if the page is open in Firefox at the same time.
This problem arose over a year ago and is related to files imported from iTunes via the media browser. If the OP will confirm that the file came from iTunes, I will have a look at the meta data to see if there's anything there that iTunes has added.
The only time I am forced to use iTunes is for apps that don't allow import from the finder such as certain situations in Keynote. Otherwise, I wish it would disappear down a black hole in the universe along with Windows and Internet Explorer.
I converted the file to MP4 with QT Pro to prove the point that its the actual MP3 file that is causing the problem - along with Safari.
I haven't depended on the QT Plugin for playing media for quite some time now. HTML5 now works in all modern browsers and should also work in IE 8 if the HTML5 Shiv is included in the head of the page.
Its also a breeze to get flexible movies in responsive web design using HTML 5...
http://www.ezmacwebdesign.com/Music-Website/index.html
I don't bother too much with flash fallback anymore since these designs are mainly for mobile devices and I can ignore those who are reluctant to upgrade their out of date browsers.