Hi I have a DOS system with a Roland CM-64 and CM-300

Here's how it's plugged

MIDI:
SB AWE64 --> CM-300 MIDI IN
CM-300 MIDI THRU --> CM-64 MIDI IN

Sound:
CM-64 Output --> CM-300 Input
CM-300 Output --> PC Line-in

Now, with this setup both module's output are mixed in the CM-300. But this also means I hear both of them at the same time.
If I'm playing a GM game, I simply turn down the volume on the CM-64.
If I'm playing a MT-32 game, I could turn down the volume of the CM-300, but then I'm also muting the CM-64...

So here's my question: Is there a way to turn off the volume of every parts of the CM-300 via MIDI using a sysex?

That would mean I'd have to do the following:
1. Turn on the CM-300
2. Play the sysex
3. Turn on the CM-64
4. Play

...But that's fine by me! I'd rather not have a switch box. The way my setup is placed, it would be as easy to simply switch the phones output from one module to the other.

Thanks 😀