Don't be too worried about your viva!
While viva is called "dissertation defense" in the United States, I would tend to see viva as an opportunity for you to share your research with people in your area. It's not so much about "defense" but more about "promotion".
Your viva examiners might be the only persons, in addition to your supervisors, who would read your thesis thoroughly. I have seen postdoctoral fellowship schemes requiring applicants to invite at least one of their viva examiners to write a reference letter. It is unlike anonymous reviewers of academic journals who would also examine your work thoroughly, but usually you don't have a chance to know or network with them.
Your viva examiners would become the persons, other than your supervisors, who know the best what you have been working on.
While I said you don't need to worry about your viva too much, I am not saying you don't need to prepare for it at all. Just like how product marketing in the industry is usually delicately discussed and framed in particular ways, we also need to consider how to frame our work in relation to the ongoing academic discussion in our fields. It is then already important when you decide whom you would invite to be on your viva panel. You would want to invite people who use similar theoretical concepts to yours, or people who work with communities similar to people who you're working with.
Then, when preparing for your viva, we should expect your viva examiners to ask questions which are relevant to the connections between their and your works. People tend to ask questions based on their own understanding of your work. And, their own understanding usually comes from how they usually approach or frame the issues they work on. Thus, reading their work and seeing what references they often use in their work are the two things we can definitely do before the viva.
For instance, if your examiner tends to approach topics from a perspective that emphasises the macrosocial structure, you should not be too surprised when they ask you how macrosocial structural forces can affect the data you have. Likewise, if your examiner tends to foreground how individuals can make their own decisions, they might ask you why you only focus on mascrosocial forces in your thesis. When your examiner happens to hold the same perspective as yours, adopt the same methodology as yours, etc., you might need to pay attention to the details of your argumentation -- how you draw a conclusion from your analysis, how your analysis is legit, whether you use statistics in the right ways, and so on.
Here are the questions I was asked in my viva. Although you definitely work on something different from my topic, some of the questions can actually be applied to different research topics.
1. Please talk about your positionality. You discussed it in the first chapter, but we would like you to talk about it from the perspective of autobiographical history. As a hearing person, what brought you to the study of deaf speech?
2. On page XX, you mentioned that some of your participants demonstrated "deaf solidarity" when commenting on the passage of disability politics. How does your positionality influence their stance-taking?
3. You emphasised a lot on your positionality as a hearing person, but the stance-taking between the interviewer and the interviewee is affected by many other factors. For example, you mentioned that one of your participant is your friend. It is for sure that whether your participant is your friend affects what they talk about. Could you comment on this?
4. It is more of a comment: you introduced lots of research on applied linguistics and disability. I think that you could also consider the parallel between disabled speakers and non-native speakers. I think you can engage with critical research on native speaker as the goal of language learning.
5. Another comment: we (the examiners) were talking about what the broader theoretical argument of the three studies you presented would be. It may be your next article.
6. Could you talk about the audience effect? You mentioned that some of the participants took part in your research wth their parents, which means that they talk to not only you but also other audiences. How would it influence your interpretation of the data?
7. You mentioned that much of the style-shifting among your participants can be considered a kind of agentive speech production in relation to you as a hearing researcher. Coud you predict what changes would take place in their style-shifting if the interviewer were also deaf?
8. methodological issues
9. theoretical framing: You applied the framework of affect in your second study, but you shifted to the framework of stance in your third study. The framework of stance actually includes affective stance. Why didn't you apply the framework of stance in your second study? Why is there a theoretical shift between the two chapters?
10. some other analytical issues