This might be more of a cassini issue with MIME Types (can you change them? It doesn't look like you can or am I wrong?).
I'm building a site that is using the audio tag to play audio files. For Firefox I'm using ogg.
When I go the to file directly, http://localhost:1455/audio.ogg, instead of playing the file just downloads. What would I need to do to get this working (without resorting to running the site through IIS)?
Try IIS Express instead of cassini, it is a lightweight version of IIS designed for development and is a drop-in replacement for cassini.
You will get better control over all aspects of the web server.