Eh, your problem might be solved easier by using the Shellexe command with a WMP command-line argument, or even better an older version of media player(MPLAYER2.exe). There is also a chance that when QDK might be messing up the code or something, because the following code worked for me with my media player of choice. You could modify your code using this snippet as necessary, and drop your mp3 into your quest directory. Would atleast let us know if it's qdk messing up.
command <play> {
shell <Mos Def - Mathematics.mp3; notactive>
}