In a question on religious experience and voices you would want to examine possible physiological, psychological or sociological explanations (just like you would in most essays on religious experience), as a way of evaluating what is called the veridicality of the experience (ie. does it refer to a genuine reality or is it an illusion?).
This clip on Youtube examines the current medical model of auditory hallucinations and shows (in a slightly comical and definitely sceptical way) the links between head trauma, other mental health issues and hearing voices. You would want to take into account such a physiological or psychological account to give a non-religious explanation of voices.
However, you would also want to look at some more sympathetic material, which doesn’t start off with quite such a materialist bias. This website has an excellent overview of different accounts of hearing voices, as well as St. John of the Cross, they have material about Teresa of Avila, Joan of Arc and William Blake. Here is an excerpt:
“St Teresa of Avila, a colleague of St John’s, noted that people who heard phrases heard such as “It is I (God), fear not” found them exceedingly powerful, calming and influential, and the memory of these voices could last for a lifetime. Such powerful and positive voices are still reported today. For example, Heathcote-James (2001) cites a contemporary account of a healthy woman, in a distressing situation, hearing a voice saying “But you have trust in God”. As a result of this, the woman described how she “felt great consolation and joy. I just cannot describe the sense that I felt, it was so beautiful it was indescribable”. These voices often seem to occur when people are in danger, under stress, or under physical or existential threat. For example, the mountaineer Joe Simpson, after a horrific climbing accident, was forced to crawl for four days back to his friends’ base-camp. During the latter stages of his agonising journey he began to hear a voice which was “clean and sharp and commanding” and which told him to “Go on, keep going”. In a biological/medical model that ignores the meaningfulness or usefulness of voices, such experiences get swept under the carpet.”