Typically, Musicmatch will not allow more than one version to be installed. Maybe you installed an older version over a newer version and confused the installation program.
"Musicmath for Windows Media Player"
Never heard of that. Is it a Sesame Street production?
Closing WMP is not an issue. It simply means that WMP will be shut down (if it is running.) MMJB is not a friendly program when it comes to file associations. It tends to steal the CD association whether it is set to do it or not. It will also play any files or downloads that it is set to play (which by default is just about everything.) I disabled all unwanted file associations and the "quick start" icon to stop Musicmatch from grabbing them. Musicmatch is optimized for streaming media so it is useful for that. There are better players for other types of files. You might also try Yahoo Media Player since it is replacing MMJB.