Correct, Rasa is text-only. Core purpose of Rasa NLU (https://github.com/RasaHQ/rasa_nlu) is user text to intent translation (with entity extraction); for Rasa Core it is mapping user intent to assistant response . Speech-to-text is one layer above these and Rasa doesn't handle it.

The easiest way to use it with voice is to connect it to a platform that supports voice input.