How would one make a MIDI to DB25 adapter?
On PCs whose game ports support MIDI, there is a dedicated UART that is set to the MIDI data rate of 31.25 kbps.

In order to do MIDI on a regualar COM port, you would have to deal with two issues: the nonstandard data rate (for a COM port) and the electrical interface, which is RS-232 instead of the MIDI current loop interface.