Well, don't mind me!
I am fascinated by this sort of thing, I've spent FAR too many hours trying to make sense of data files, mostly work related involving data files from accounting systems, etc.
I've been looking in some detail at the .SNG files provided with the installed system, specifically MINUETG and the Joplin one.
The data at the beginnng of the file looks like nothing sensible, but much further through I come to large blocks of data, each block headed with what must be a track name, and these blocks start looking a bit (and only a bit) like midi data. BUT, there does look like there could be some structure to the data, a bit like midi. It would certainly help to have a midi file, and a .SNG file, of the same thing (supposedly).
I've downloaded a manual for version 4 of Voyetra - OK, it's the manual for (I think) the Atari version, so the screen pictures don't all tie up, but the commands etc seem to be the same and the manual should be useful. Found one thing, the MODE command refers to swapping between SNG mode and MID mode etc, and supposedly the system saves using the current mode, so does that mean that if you LOAD a .SNG while in SNG mode, then swap mode and then save while in MID mode you save a midi file? Not found any sign of a better way, and you cannot (it seems) change the file extension, this is totally dependant on the mode you're in!
By the way, the system does seem to perform fine on my Pentium 75 running DOS 5.0. I've now loaded the driver for my Roland LAPC-I card, but I've not tried playing anything.so I might trouble to see what midi actually does.
I've not seen any sign of any utility progs to convert SNG to MID, but I have seen reference to different varieties of SNG, incl some that are essentially digital audio (were these to do with Korg systems ?) so you'd need to be careful. I've had the same problem looking for info about .MUS files, there are different versions of that as well.