If they really are MIDI files, then if you change the file extension to “.mid” they will work in any program that can play MIDI files.
If that does not work, then either the files are corrupt, or they are not MIDI files.
If they are not MIDI files, then you may need to obtain the original software / hardware that created them.
That’s a shame, because you could probably have used that.
If you search the Internet, there is a demo version of MasterTracks Pro 6.8.4 for Windows available as a free download. You could try that and see if it allows you to convert the MTS files into proper MIDI files. Most of the download links that I found look a bit suspicious, so be careful and ensure that you thoroughly virus check anything that you download. This looks like the most “official” download location that I could find, but I’ve not tested and security of your computer is not my responsibility http://sur.ly/o/gvox.com/AA001290