Final answer:
To ensure the voice reaches the uttermost parts of the theatre, a performer must project the voice, which involves using proper breathing and speaking techniques.
Step-by-step explanation:
In order for a performer to throw the voice into the audience so that it penetrates to the uttermost reaches of the theatre, the performer must project the voice. Projecting the voice involves speaking from the diaphragm and using correct breathing techniques to enhance the power and reach of the voice without necessarily speaking loudly. This technique ensures that even those at the back of a large venue can hear the performer clearly. While increasing the room temperature may somewhat increase the speed of sound, and raising the pitch might help the voice carry better, these are not the primary techniques for ensuring voice projection in a theatre.
Moreover, using a microphone is indeed helpful, especially in big venues or when musicals are performed, but the question specifically refers to the performer's own ability to project. Whispering would have the opposite effect, as it would greatly reduce the reach of the voice. Hence, voice projection is the key, not merely speaking loudly, which might result in a strained voice or a lack of clarity.