Errm, no you don't. You may think you do, but in reality what you need to define is what you mean by multi-threading.
That was a provocative opening paragraph, but it's important for you to realise that you need to put bounds around what multi-threading means in the context of the application. If you are just talking about converting the audio into text and not having it run on a UI thread, then you would simply want to run the entirety of the process in a background task. This is fairly straightforward (I would do this using the Task Parallel Library).
Task.Factory.StartNew(()=> ConvertAudioFile());
If, however, you are talking about converting each line on a separate thread, then I would caution you against this for the following reasons:
1. How do you know when a line has finished in an audio file?
You would have to process a line to know that it was a line. So, what would you gain by putting it on a thread?
2. Without complex synchronisation, you aren't guaranteed to get the lines out in the order you put them in. That's kind of the point with threading - it's not deterministic.
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