I'm not entirely sure, where to put this, since it seems to be a technical issue as well as a story related one.
This time round I've chosen the more evil path of letting the vault continue their research and Covenant joined the minutemen. After I while I got the message of Covenant being under attack. And after that attack, things are on the Fritz. Every named citizen of Covenant is suddenly mute. They gesticulate, but their lips aren't moving and there's no voice.