First, you're welcome!
Second, I have my answer re: your question about voice being employee involvement. And, that is what I discuss in my leadership development programs with senior leaderships and C level executives from client organisations. But, I will not answer your question. Not because I want to make it difficult for you. Instead, I want your dissertation to be a rich and invigorating experience. Because, you've decided on a topic which is extremely powerful and almost entirely separate from most run-of-the-mill leadership dissertation topics.
Third, regardless of which leadership style context Voice is analysed in, the outcome is always the same. The only time it will vary is when it is applied in the context of management as opposed to leadership.
As for researching in a 'live organisation', my suggestion (given your choice of topic) is to select one that has existed for at least 25 - 30 years. Because, it is only in sustainable organisations that Voice is truly visible as a couse of ordinary business culture!
Once again, good luck & have FUN!