Apart from the Yamaha software cartridges there’s also some specific 3rd party CX5M software available. One of the most productive persons who was developing software for the CX5M is probably Carolyn Eng who produced one of the most useful software pieces for the CX5M: MidiSlav.
MidiSlav is a piece of software which turns your CX5M into a FM-MIDI Module comparable to the FB-01. You can setup the 8-voice polyphony across different MIDI channels or unison them together. When I bought my CX5M-II There was a copy of MidiSlav 1.4 to go with it.
There’s one thing: MidiSlav being a disk program is a hassle for real CX5M users (It also was a challenge to get it off the floppy and putting it on here). Most of the CX5M users have no disk unit attached and can’t use MidiSlav.
It would be much more convenient if the code would be available as a .ROM image so putting it on EPROM would enable any CX5M user to use this fine piece of software. The code would have to be rewritten though.
The disk contains the source code. I believe it’s compiled in GEN80. If anyone is skilled enough to convert the code to a ROM-type boot it would be very handy. Please let me know if you’re interested in working on this. I have Carolyn’s blessing on this, So no problems there from the original author.