Hey, guess what! I have a Roland SC-88VL and a gameport too 😀

What you want to do is connect the OUT connector on the Gameport cable to the MIDI IN A connector on the SC88VL. You may as well also connect the IN connector to MIDI A OUT/THRU, but this step isn't really needed.

You'll also need to configure your OS/game to route midi to the external device. It sounds like you already did this, but I may as well provide instructions 😀. I am using an AWE64 Value in my machine, so in Windows 98 I needed to run the 'Multimedia' item in Control panel, switch to the MIDI tab, and select "AWE64 MIDI Out" as my output device. In DOS games, as long as you AREN'T use AWEUTIL to emulate GM with the internal synth, it's usually enough to select General Midi in a game's configuration options.

Finally, you need to mix the audio back together. I have an AUX in on my speakers I use for this purpose, or you can use a line in on your sound card, if you can get playback this way. Or you could put the sound card output into the IN port on the SC88VL then put the out to your speakers. Or amplifier. Or whatever.

All that said, if you hear ANYTHING, you probably have it hooked up correctly. There could be some additional things wrong.

1. Your RCA to Stereo Mini plug cable could be faulty in one channel, assuming you are using one and your speaker/amplifier doesn't take RCA already. Try another cable if you have one
2. It could be an issue with the SC88VL settings. Try resetting your SC88VL. Hold "Part >" while turning the power on then press [ALL].
3. I've also had cases where a part got misaligned with its midi channel, possibly by the previous owner or by messing around with the buttons 😀. A reset should fix this, but if not, make sure you are not in ALL mode (light is off) and press the Part < or > keys to cycle through the parts. Make sure the MIDI CH entry on the display matches the entry for the selected part on the top left. If not, use the MIDI CH < or > buttons to re-align it
4. If you have your MPU-IPC-T and the Sound Card with Gameport on the same PC, they could be conflicting. Please make sure they have different IO Ranges (e.g. don't put both on 330).

Thank you zerker for your help!

Works! I had it right the first time around!
I still don't understand why there are 2 soundtracks, 1 with an organ playing on channel A and one with a piano on channel B.
Both are the same, except for these 2 different instruments, so I thought I was missing a wire to channel B also.
After checking with a DB51 and a video on Youtube, channel B is apparently not used, at least for games. Am I correct?
Also, if I plug the connector that says "In" in the "Out/Through" plug on the SC, Doom 2 becomes very choppy ans slow.
A whole new chapter of quirks opens before me! xD

Thanks a lot, I'm one happy puppy!

I'm surprised you have an adaptor that gives you a channel B. I expect that's for very large midi compositions that would require more channels than normally available on one connector. Yeah, games don't use it.

As for the Doom 2 thing, I noticed the same thing, but didn't realize that might have been the cause. The In plug is designed so you can send MIDI data back to the PC. If some games get confused by this, maybe it is best to leave it disconnected. It's really only useful if you have a MIDI keyboard to connect to the sound module and want the midi data to ALSO go to the computer.

I may have to re-test some games without the "IN" cable connected.

Sorry, I have expressed myself poorly.
I don't have an adapter with both A and B channels, but a normal one with just one output and one input.
I turned off the SC, unplugged the wire from A channel, plugged it in B and turned the SC on again.
There I noticed different instruments, freaked out, and spent all afternoon looking for an adapter with two out connectors to plug in both A and B channel...
After a couple of hours scouring the Internets, I finally decided it was best to ask people who know their stuff. Am I ever glad I did!