Voice
Talking with the agent, and having replies read aloud.
A live call
The button is in the row of buttons under the message box, next to the paperclip (attach) button; one click starts a live spoken conversation with the agent, and the same button ("End the call") hangs up. The call uses credits while it runs and ends when the credits run out.
Dictating a message
The microphone in the message box turns your speech into text. Hover over it for a moment to find Auto-send.
Hearing a reply
Read aloud is the speaker icon in the row of small icons under a finished reply.
In Account Settings, the Voice & Audio card has Auto-read responses and Summarize before reading. Both are saved in the browser you set them in, not on your account, and reading aloud uses credits.