I looked into it. And all my voices seem to turn off and on just fine. If you tried the muting of MC BEFORE you choose u name. It will play. Once the make your name it mutes. Thats because how renpy characters works in coding.
Tested it myself, but the voices can be turned off as soon as the Commander shows up on screen at the start of the game, before naming. It's just the unnamed prologue bit with the spaceship background that can't be deactivated by a toggle (nor the NPCs, such as the Saboteur that finds the Commander shaking off the cryosleep). In other words - turning off the Commander works great, even before he's named; turning off the unnamed NPCs and the untitled announcer at the start does not (presumably because there's no toggle coded for them).
That being said... one more toggle for "other characters" might be an easy way to set it up, if people just want the named characters to be voiced - just attach anything that's not coded for one of the toggled voices to that last category (including the prologue bit).
In all honesty, from what little I know of Ren'Py coding... going back through to tag all of those would be a living nightmare - and anyone that's going to go as far as wanting to mute that much of the voice acting is probably simply going to use the voice slider to mute the lot, like I did. My original post was just to bring up a potential bug, but I'm all sorted - thank you for looking into it, and thank you for such an auspicious start to a promising game! We're eagerly awaiting more (though, as stated earlier, real life comes first - it's worth the wait). <3
Sorry for the late response XD. I read your post and had my wheels turning a bit. How renpy works with how I did voices there is a voice tag. I think you may have a good idea here. I could have all minor characters under the same voice tag to mute them all. I have jotted this down and will make that change when I can. Probably wont be for a patch or 2 as itch updates lags behind for marketing purposes. So yeah, thanks for the feedback :D