All VoiceThreads are completely private by default. The person who creates a VoiceThread is the only person in the world who is able to see it. The only exception is content created by students under a K-12 license. Click here for more about student oversight for K-12 educators.
A VoiceThread remains private until the creator makes an overt sharing action. That person always retains control of who else can see the VoiceThread, whether that is an individual, a Group, or the whole world. Learn more about sharing here.