It's required that Dialogflow agent is set up to support API V2 (since API V1 doesn't support streaming) and Beta features should be enabled (Settings -> Enable beta features and APIs) since speech synthesis is in the beta features list at the moment.
Now create and download the service account JSON file associated with the agent from GCP console (please read the Setting up authentication article for details). "Dialogflow API Client" role should be chosen while creating the service account. The JSON file is required for Voximplant to be authorized before sending audio data to the agent.
Click on the Speech tab in the agent settings to set up Speech synthesis options.
Enable Automatic Text to Speech by clicking on the toggle button, choose MP3 or OGG in the Output Audio Encoding dropdown and one of the available voices - we highly recommend to use WaveNet-powered voices since they sound much better than standard ones. Save settings by clicking the Save button in the top right corner.
Only MP3 and OGG are currently supported