What's 'original voice'?
'Original voice' is the term used by producers/execs/script editors when describing a writer's particular style, or lack thereof. e.g. 'This script has no original voice' or 'we need scripts with more original voices'.
A writer who demonstrates 'original voice' will have a distinctive style or a fresh storytelling approach, something which makes their scripts easy-to-read, original, entertaining and/or thematically resonant, or all of the above (for thematically rich writers, they may be referred to as someone who has 'something to say'). See Jimmy McGovern, Paul Abbott, Russel T Davies, Tony Jordan, Howard Overman, Ashley Pharoah, Matthew Graham etc.
For more on original voice, see this post from 2005.