Are they subtitles or closed captions?
If subtitles, they are images in a vob stream. They can be extracted (generally to an index of images and a file with the images). OCR (optical character recognitions) is required to get them into a text format. And any OCR operation requires editing for errors.
Closed captions can be extracted as text.
I have not tested any of the methods.
2 videohelp threads that discuss this:
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