Hey I been trying to get this work and have noticed there is an error with the WMPSTOP code, whoever made it has just put the code for the next command instead of the proper stop command. The correct code is 18809. Also you don't need a script to auto open media player if it isn't loaded. Instead you can put the path to mediaplayer as the last argument in your *Send definitions this instructs remote to open the path if it can't send the message. Here's what my WMPSTOP definition looks like

*Send WMPSTOP "WMPlayerApp" 18809 $AudioPlayer$

Hope this helps anyone who was wondering why stop was behaving like next.