In our line of work, that's the million dollar question - which do people refer to it by and how do they search online? The world hasn't come to any kind of mutual agreement or consensus just yet... However, at FlyVoiceovers, you'll see that for search's sake, we've used all versions throughout the site - which disturbs our inner sanctum somewhat and plays havoc with the ol' OCD, but it is necessary with our clumsily-christened industry.
Search volume on Google's Keyword Planner suggests most people search the broken or hyphenated iteration (2015). Although I think we can safely assume that by the numbers, Google reads the top two as the same search term, so you could potentially split that 27,000 average monthly searches up between them for a more realistic figure for each:
If you regularly seek VO (played it safe there), what do you find yourself commonly searching for? And if you are a voice talent, which term is prominent across your website and profiles?