Robin Good: Hangout Captions is a new experimental app for Google Hangouts that allows you to add a real-time text transcription of what is being said inside the Hangout.
The text transcription is displayed next to the event allowing deaf or hearing-impaired participants to read and understand fully the topic being discussed.
"“By adding the +Hangout Captions app, you can either connect live text from a professional transcriptionist to your Hangout, or type right into a text box yourself to transcribe a Hangout for your friends,” Naomi Black, technical program manager for accessibility engineering wrote in a post on Google+."
"You’ll only be able to see a transcription in the Hangout Captions side panel when someone in the Hangout is providing the transcription; either through live stenography (CART or palantype) or by simply typing into the app. Live transcription isn’t automatic."
"Hangout Captions is an app that connects live transcription services directly into a Google+ Hangout, improving accessibility for participants who are deaf or hard of hearing."
P:S.: In late May, Google added the ability to add captions to videos uploaded to Google+.
Support Forum: https://groups.google.com/forum/#!forum/hangout-captions
More info: https://hangout-captions.appspot.com/