To anyone confused about the voice.
There is a very important detail in this movie. One that is deliberately hidden by the director but once found enhances the whole movie. It explains the voice that Coke keeps hearing and also explains the strange mannerisms and behaviours of the people from the future.
.....Everyone in the future is schizophrenic!
The film starts with a quote from a schizophrenic person and later we are told that only approximately 1% of humanity survived the virus. Well approximately 1% of the human population is schizophrenic! So the voice that Cole keeps hearing is his schizophrenia and the reason that everyone in the future acts weird is because they're completely nuts!
Hope this helped clear things up for anyone wondering about the voice.