The subject of my dissertation was The Use of Sound Effects and Stylised Ambiences in Filmmaking which turned out to be a really fascinating focus of study. It was, however, a huge can of worms that I could only touch on in the required word count. I think the most important thing is to find a subject that you are passionate about and one that will have enough background material to support your writings. I'm not sure if things have changed much but when I did my dissertation it was all about your references, so bare this in mind when choosing what to write about.

At the time, my work method was to initially brainstorm all the ideas I could think of that were related to what I wanted to study. After a few sessions of refinement I had a clearer idea of the general subject. I then set about trying to plan how I could translate these ideas into different chapters and to get a general structure worked out. Once this was decided, I began writing, but not at all in order. If I remember correctly, I began writing a chapter which turned out to be the fourth or fifth in the final dissertation, then I did the third, then the last and so on. It was only really in the last couple of months that I began working all of the chapters together to form one piece.

It might not necessarily be the way you would approach it, but it's worth baring in mind that although the final piece will be a linear writing, your methodology doesn't have to follow this.

I think those mentioned are already very large topics and there's a slight danger of subjectivism (unless you focus the dissertation just around the sound piece that you produce yourself). For example, is "the advances of sound used in modern day sci-fi films" supposed to focus on artistic or technical advances? And how is artistic advancement defined then? I would narrow the topic considerably, because one could really write a book about either of those topics.

"Audience perception of sound design in film" is a good topic, but I think research on that area has been done to death long ago. You should also narrow it down to a specific film style or a specific group of film goers for example, because the topic is otherwise too large. For this topic you could find a lot of sources and conduct a modern questionnaire to test those hypotheses in the genre and in the specific group of people you've chosen.

Any area, medium, style or technique you're particularly interested in film/TV/game sound design?