If you were considering a weight loss course you may be forgiven for believing that the program lacked credibility if the person running it was significantly overweight. The same could be said for a voice coach that has a squeaky and irritating voice. It does not matter that this person has helped lots of people lose weight or have a wonderful resonating voice. You assume that unless it is working for them it will not work for you either.
What about coaches that do not get coaching for themselves or are not open to change? Or the person who trains people in communication skills and yet cannot get their message across simply?
People often make their mind up as a result of how things look and sound. Whether it is right or wrong is immaterial, it is human nature and you are more likely to succeed if you go with human nature. We want to see our doctor wearing a white coat and our accountant wearing a pin stripe suit!
Obviously if your target audience prefers to shun social norms then that is great but it is still useful to check that you are not putting off your ideal audience by the way you look and sound.