Sound is connected to the function of listening, it is associated with the ether element and Vata folks are the ones that have the most acute hearing. It is claimed that inner speech and speaking out loud share the same brain mechanisms (Broca's area). As you can imagine, scientists interested in understanding inner speech, and the extended version of "voices in the head" have found it difficult to simulate a study that would assess spontaneous thoughts, since they are...spontaneous.
The one simple thing we know about yoga is that it has taught us about the idea of brain wave movement and associating thoughts (and thereby speech) with the frequency of movement. They have also determined that movement is determined by the function of air - a lot of air makes a lot of movement. So breathing became the medium through which they determined could change the movement of thought/inner speech. They figure if we control the breath, we control thoughts.
We could do well to teach this idea in schools. We could also do well to check in with ourselves habitually - if our breath is not calm and measured, can we really trust the inner speech going on in our heads?