Facebook has been known to roll out experimental or new features gradually to users before a mass rollout to everyone. We saw that with the move to standalone messenger app as well as the stickers where the roll out was pretty limited to the privileged few. A new feature seems to be here once again and few people have already reported getting it.
The feature basically converts your voice messages in the form of a text message and writes it below the incoming voice sample. The reason for doing this is that in case somebody has sent you a voice message and you are in a public situation where you cannot listen to the voice message you can simply read the message and send across the message appropriately.
Given that Facebook has already rolled out this feature, it is not beyond the realms of possibility that Whatsapp will bring something very similar in the upcoming days given that Voice Messaging is really popular over Whatsapp too. The feature is set to roll out to all the users of the messenger app in the form of OTA. It is also not clear how accurate and perfectly does this work given how much of an effort voice to text translation is currently.