In an official post on April 14, Discord revealed that all users of the mobile app can now send voice messages up to 20 minutes in PM and servers. Messages can be listened to on a computer, but for now, the ability to record and send them is limited to the mobile app.
It works in much the same way as the voice messaging feature in Signal, WhatsApp or any other mobile messenger, with a new microphone icon to the right of the text field – just hold it down and recording will start.