Convert MUSX to MUS

Both .musx and .mus file originate from Finale music notation software, which means that this software (namely version 2014 or later) is most like your best choice for musx to mus conversion.

Everything points to the fact that the music notation export from newer format (.musx) to older one (.mus) is doable, because Finale 2014 introduced backward compatibility and since this versions users can export musx to mus using the File ► Export ► Finale 2012 function.