Your tone of voice can affect your client's ability to hear you, especially in times of stress. In order to have a tone of voice that allows us to be heard, we need to monitor our own level of tension, our own level of defensiveness, our own level of feeling safe. Ongoing monitoring requires mindfulness.
And a truly calming tone of voice is very different from a maneuver, move, or ploy to manipulate a client.
Dr. Stuart Shanker at the Web site of the Canadian Self-Regulation Initiative explains part of Dr. Stephen Porges's Polyvagal Theory and then recommends monitoring your state of tension because "[a] very basic contagion effect operates here: tension in the speaker begets tension in the listener." A tense listener is not an accurate or receptive listener. (Although Dr. Shanker is talking about children, monitoring one's tension level when talking with adults is also of critical importance.)
The tone of voice that facilitates good communication does not result from a technique: it flows from the speaker's state. It comes from an inner feeling of safety and conveys that safety to others. Porges's Polyvagal Theory provides the neuroscience and physiology behind the effect of voice tone.
If you want to interact effectively with clients, I highly recommend that you take some time to listen to Porges talking about his theory here or here (slides for the CCARE talk). (Other talks are here.) I have found that listening at those links was easier to understand than reading his book or articles about his work, with the exception of this written interview and this one which are easy to understand.
Once again I am featuring wise practitioners, Shanker and Porges today, who do not talk about stuffing your bag of tricks fuller or