The term 'Voice' referred to an audio-neuro control mechanism that enables the manipulation of speech to achieve complete control over a receiver It is perfected by the Bene Gesserit through the combined training of several advanced techniques.
Application of Voice
Use of Voice allowed a Bene Gesserit adept to gain control over uninitiated victims merely by altering the tonal qualities of the voice. Simple instructions or orders were then spoken in Voice and obeyed dutifully.
Voice was most often used to compel a typically unwilling person to do something undesirable or to gleen the truth from an unwilling recipient.
Resistence to Voice
With sufficient natural ability and training one could become more resilient to the effects of Voice, as was most famously seen with Paul Atreides. Other methods of resistance came through technology, such as the Cone of Silence.
Possibilities of Its Use
Bene Gesserit used Voice for interrogation, as well as subtle manipulation.
Jessica Atreides had considered the possibility of using Voice on the Duke Leto Atreides to make certain decisions regarding the welfare of House Atreides upon their move from Caladan to Arrakis. She had surmised that she could use Voice in the most subtle ways imaginable, so that the Duke would think that he was making such choices of his own volition. However, The Lady Jessica knew that were she to do so, it would be a gross violation of his trust, even if it meant that they would save House Atreides in the process.
Some people had the ability to use Voice without formal training. Sheeana was an example of this. Through Bene Gesserit training she learned to master this natural talent and she grew to adulthood.