The error message is telling you that the file you're trying to load does not exist. type //run $mircdir in mIRC (with two slashes) and make sure you copy the file to the folder that opens up. Then you can re-type the command.
As a sidenote, you don't need a script to play mp3 files in mIRC, simply use /splay. Type /help /splay to read how and/or use /splay -p file.mp3 -- file.mp3 is a relative path from $mircdir. You can also use //splay -p $sfile(*.mp3) to pop up a file dialog.
