Brief Answer:
This could be due to anxiety or hallucinations.

Detailed Answer:
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Hearing voices in head could occur in a number of conditions. One is, in severe anxiety. Sometimes in severe anxiety individuals hear sounds in head. These are sounds like running machines or could be ringing in ear.

Some ear related causes could also result in such symptoms.

Hearing clear sounds or voices occurs most commonly in some psychotic conditions. Psychosis can result in hearing voices like voices commenting or voices discussing etc. These are seen in auditory hallucinations. These voices are not heard by any other individuals.

Please provide me details what kind of voices you are talking about.

Thanks

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