There are several problems that creep in when trying to do what you are asking.
First, the web server sees and recognizes the type of file (with MIME type and using the 3letter extensions).
Your web-browser recognizes the 3-letter extensions and MIME type sent by the webserver.
For you to download rather than 'play' the items you can change your MIME preferences in YOUR browser to not automatically play files of that type.
BUT it only works for you. If you want ANYONE coming to your site to be able to do this, you would want to change your webserver settings and the MIMEtype to force anyone to 'save' the file rather than play.
Sorry this doesn't directly answer your question, but it may point you in the right direction.
If anyone has a better explanation, please chime in.