In a photography contest that I want to take part in, the theme is 'Voices' . What does this mean? How do I portray this? There is nothing given apart from the topic.
If you are set a theme or topic for photography, it's often difficult to know where to begin, especially if it's a single word.
Start with the first thing that comes into your mind when you were first set that task, or when you first read about that competition. In your example, 'voices', what came to mind first? Was it an image or a scene? Can you recreate that? If you don't know what the word means, look it up in a dictionary. As you read the definition, what comes to your mind at that point?
The whole point of single-word topics like these are because the people that set the task are looking for unique interpretations. 'Voices' could be a choir, it could be political voices, as mentioned in one of the comments, it could be a record player, but these are just the most simple interpretations. What about voices from the point of view of mental health and mental care? What about the record shop HMV in Britain which stands for "His Master's Voice" and is represented by a dog at a gramophone?
Think about the word and where you've heard it said and the context you've heard it in. Think about it in the context that you would use it. Think about the emotion that comes to mind when you hear that word and try to reflect that emotion in a photo that you think portrays that word.
As a photographer and artist you have the freedom to interpret a challenge or a set word as you wish, so be creative. The judges may be looking as much for a creative interpretation in a response as they are for technical skill.
I ask a lot of questions here and I don't explicitly answer your question as it stands at time of writing, but I'm trying to approach it from a more generic point of view, more in the sense of 'if given a topic, how do you go about portraying that topic?', which is probably a better fit for the site.
My answer would be that it is not for anyone else to answer this question for you, Suggestions for you to think about, Perhaps.
The theme is for The individual photographer (you) to interpret and to successfully communicate to the viewer in a visual medium by way of execution of composition, subject matter, exposure etc.
The ability of the photographer (you) to interpret and communicate the theme is one the reasons to have a theme in the first place.
I would argue that the ability to communicate the theme is of equal importance to the technical execution of producing a good photograph and is the reason to have a theme in the first place.
In theory the viewer should be able to look at your photo and recognize the theme ( or what you are trying to say ) even if they do not know what the theme of the contest is, and if they do know the theme then they should certainly recognize the theme in your photo and FEEL that you have expressed it.