To caption a previously existing VoiceThread make a copy of the VoiceThread (to create a new one) and that will trigger the automated captioning process.  To learn how to make a copy of a VoiceThread please see this link for more information. https://voicethread.com/howto/copying-a-voicethread/ 

New VoiceThreads are automatically captioned.  Please see this link for more information. https://mizzou.freshdesk.com/solution/articles/11000070787-voicethread-closed-captioning