Bottom line: Microsoft said either of these features (or both enabled at the same time) can help deaf or hard of hearing gamers or those who cannot speak participate in Xbox Party Chat without any special accommodations from other members of the chat.
Starting today, Xbox Insiders can try out a couple of new Xbox Party Chat features. When enabling speech-to-text transcription, all words spoken by people in your party chat will be converted into text and displayed in an adjustable onscreen overlay.
Conversely, enabling text-to-speech synthesis will take your typed words and have them read by a synthetic voice to the rest of the people in your party in your choice of voice.
Microsoft has offered similar functionality in Game Chat for a few years now, but this is the first time the functionality has been applied to Party Chat.
Microsoft notes that the features can also be useful for diagnosing microphone problems or differentiating game audio from chatters.
Insiders who want to try out the new functionality can head to the settings menu and look for the Ease of Access section under Game and chat transcription.