OK, I just received the ROLLS mini mixer, and I have followed your connection suggestion and this is what I have now:

KNOB 1: controls the volume of SC55 soundtrack music of a given game

KNOB2: controls the volume of MT-32 soundtrack music of a given game

KNOB3: controls AWE64's sound effects volume, meaning that I can completely turn off the sound FX volume of a given game!

KNOB4: yet to be connected to UM-ONE and to be tested out

So far, so good, but what I don't understand is this:

1) Where does the AWE64 LINE-IN go to? Nowhere at all?

2) I was always under the impression that:

* The game code sends out the MIDI (and the sound FX) instructions via Joystick MIDI-OUT
* SC55 (or MT-32) receives this code and processes it
* This processed data is sent BACK to AWE64 via LINE-IN (AWE64's LINE-IN)
* Finally, AWE64 LINE-OUT, sends music AND sound FX to the speakers

3) Again, it seems that LINE-IN of AWE64 can be left out, and connected to NOTHING!

4) I don't like the fact the I can adjust the volume of AWE64 sound FX (connected to Knob 3 of ROLLS)

Your insightful input is appreciated as always!

A bump to the above situation?

the mixer is doing what you used the line-in for
there's no data being sent back to the computer if you connect to module to the line in, just audio, and the sound card mixes it together with the FX so you can hear both on your connected speakers, you're just bypassing the sound card mixer now by connecting everything to an external mixer directly.

Please see below post!

OK,
It seems that the LINE in under the Mixerset applet of Sound Blaster was all the way DOWN! As soon as I raised the volume, I get audio!

Here is my new setup:

MT-32 Line OUT ---> ROLLS IN
SC55 Line OUT ---> ROLLS IN
ROLLS OUT ------->AWE64 IN
AWE64 OUT ------->Speakers

When I launch an MT-32 game, I just raise the ROLLS knob associated with its input, and keep others muted as suggest by darry, same with SC55 Sound Canvas / GMIDI games.

darry was nice enough to mention that the extra noise from AWE64 line IN might be something that I should consider, henceforth I am inclined to use his original suggestion.

Not getting a sound from the LINE In of AWE64 was something that was really annoying me, and it was some sort of personal quest to solve that issue! Now that it is solved, I feel relieved and can use darry's suggestion.

One last question:
In your forum suggestions, you mentioned an optional MIDI thru box, which would eliminate the need to have all the units being ON at all-time.
Is ROLAND MT-104 considered a MIDI thru box? Because I have found one in pristine condition on eBay (although pricey, around $125 USD)
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no, the MPU-104 (what I think you meant) is not a midi thru box, it literally says on the box what it is "midi input selector", you can switch between several inputs and go to one output, that's not what you're looking for.

midi thru boxes can be had for under $50

ROLLS OUT ------->AWE64 IN
AWE64 OUT ------->Speakers

why? this is kind of an unnecessary step going through two mixers
just connect the AWE64 out to the mixer in and control everything in there and go to the speakers, you can still control the FX volume on the card and you can leave line in muted for less noise.