How Can We Hear The Voice In Our Head When We Are Thinking?

Hearing happens in the brain, and our brain is capable of recalling and synthesizing this experience regardless of external stimulus.

When we think our mind "brainstorms" ideas that we are able to share and express using our mouth, thus people can hear about it. When this "voice" in our head is "speaking" nobody else can hear so ultimately we are just thinking over what sound and voices sound like. So in the end we don't hear it, we are just thinking it.