The length of an Audio tweet is limited to 140 seconds. You talk longer, it is automatically created a Thread that up to 25 of such a language-Tweets include. Voice messages can only be as Original Tweets published, and Audio-responses to the Tweets of others are not possible.
For the time being only available for iOS
Twitter wants to deliver the Feature to a limited group of iOS users. In the coming weeks, it will be used for all Twitter users with iOS devices. If and when the function can also be used with Android devices, it is not clear from the Twitter communication.