The importance of adopting the "correct tone" was clearly illustrated in the Brad Lau saga when Brad responded to allegations against him in a "calm and objective manner" (http://cwfong.blogspot.com/2010/08/brad-lau-saga_24.html). This response is uncharacteristic of an innocent person (as supported by David Lieberman's research), and thus worked immensely against Brad in his efforts to contain the crisis.
In addition, a communication study by Albert Mehrabian has shown that a lot of communication comes through non-verbal communication and when we are unsure about words and when we trust the other person less, we pay more attention to what we hear and see.
In short, in a crisis communication situation, PR Professionals must not only ensure the accuracy and appropriateness of the words used but also the "tone". This is because the unspoken word is as important, if not more, than the spoken ones.