All grounds tend to be connected together, same for the same level voltage lines.
for midi, midi in, midi out, ground & 5v. BUT ! one, that only works on a soundcard which has midi support on the joystick connector & two, you'll still want/need an adapter for adapting the midi signals, you know, one of those things with an optocoupler in, so for that you'll need a 5v line.

Which leaves you four button inputs and four axis inputs, for which you'll also need a ground and a 5v.

No idea where you're going to with the 6 buttons though.

Four sound cards in different machines.
All from Creative ->
CT4180
CT3670
CT3780
CT2910

One Chill & Phil adapter with attached MIDI module (SAM Wave).

You could get General MIDI Music on any system by simply replugging it externally.
But i still need my joystick or gamepad.

My solution was this:
https://www.serdashop.com/DB15-Doubler-Solder-Kit

Only once again soldering didn't work because I just can't solder. I sent it to someone who could solder, but then it got lost in the mail. Yeah.

And now I'm just looking for a replacement that I'll have to make myself because the shipping costs from Belgium are just too high.

It took forever, but it was still delivered. Was soldered and is already back.

To keep it short, despite mechanical inadequacies and wiring problems, it works. 4 button joystick or 4(6) button gamepad and MIDI simultaneously via the game port. Just in case anyone ever wants to know if it even works. Yes, it does! 😉

I never had any doubt about that working on most soundcard gameports.

Maybe in theory. However, the number of people who use it in such a practical way is likely to be very small. Until I hear otherwise ~1. 😉