Thanks for mentioning
I did actually find this one, but my problem is that it does not explicitly grant permission to use the audio files for commercial purposes. Maybe I'm only overcomplicating this, but since Balabolka is using the Windows SAPI and the Windows voices which is to my knowledge is free to use in a Windows application but because I do want to use the voices also on Linux, Mobile and in the browser, I'm not entirely sure if that would be ok to do. In case what I'm working on is going to be a success, I don't want Microsoft coming after me because I did not have the rights to use the voices outside Windows. And the some goes for all voices I can install. It does not mention it anywhere if I can use those voices in such way for commercial purposes 
Again, maybe I'm overcomplicating this, but I would prefer something that explicitly grant me permission to use the audio files in commercial projects. But Balabolka is definitely going to be perfect for making placeholder voices and then if I'm still uncertain regarding the legal part of using the voices, I can replace them with ones coming from Acapela Box 