Our voice is the indicator of the state of our total health - the mind, body and spirit.
It changes as we change and our environment.
Sorry for a vivid example, but when the men are castrated, the pitch of their voice changes.
When I'm upset or shy, or the air is dry - you will immediately hear the difference in my voice. Btw, so does my piano - as the weather changes, so does the piano's tuning.
My point is - nothing in our whole being is constant - the heart, the skin, the kidneys, our mind and so on - all vibrating and adjusting to the situation.
We can observe ourselves on how we react to these changes and make adjustment as needed.