First thing: I don't recommend you use it on an internet-facing web server. I just threw it together for my own mollification. It is not hardened for internet use. Let's face it, it's accepting local pathnames as parameters. It's not internet-safe.

CGI scripts frequently go in the cgi-bin directory of your web server or wherever it's configured to look for CGI scripts. Also, this particular script assumes that you are on a Unix-based system (the mplayer command invocations are Unix-specific).