I had the same problem few days ago. This was caused by google, because google added/changed/deleted voice sources. You can check this with the developer app:
This standard voice is used if you don’t configure a special one in the app card. You can test if your configured standard voice is still active by selecting the flow card with the voice option. If the catalog of available voices doesn’t include your standard voice, you have to choose another one out of the catalog as your new standard voice an save it.
