Both .cmt and .cmtx files are commentary data from bible study software called e-Sword and most likely come from different versions of the software, with the .cmtx files being introduced in version 9. This means that version 9.x of the software can be probably used to resave older .cmt files to the newer .cmtx files and there is actually the eSword 9 Converter for that.
Updated: October 4, 2023