Thanks for checking out the game :3 As far as I can remember, you'd have to turn off all SFX to get the voices muted as well. Don't quote me on it, but I think you can still keep the BGM volume up. It's been so long that I can't quite remember though >.< I just remember the fact that at the time of making the game, Tyranobuilder (the engine I used to make it) had no option built-in to let a dev differentiate voice acting from other SFX, so I think it all had to go in the same category, unfortunately!
It's one of the many reasons why I wound up switching to using a different engine in the end, cos with what I use now, not only can I put the voice acting on a separate channel to SFX, but I can even easily add individual volume sliders for each character, which is sooo much better! But yeah, it's not an option in SR, sadly :(