Determining the 'amount' of speaking in a video
The most straightforward and easy thing you could do is rectify the signal (i.e. take the absolute value of its samples) and then run a moving average filter to get an approximate estimate of the "envelope" of your signal. Imagine the moving average filter like estimating the mean of the signal values between samples 0-100, then 1-101 then 2-102 and so on. 

The result of this will be an inaudible slow varying waveform. You can then do a histogram on its values and use that to estimate a threshold which could then be used to "tag" the interesting parts of your video.

However, I feel that this is a lot of hard work for this kind of problem.

Therefore, I would also like to suggest that you use a video editor that presents the video "signal" as two separate ribbons, one for the actual video and another representing the amplitude of the sound. You can then quickly inspect the audio ribbon and estimate quickly the parts that could be seen as "interesting" and use the video editor to either directly transcribe those areas or cut the video down into smaller segments. Unfortunately, at the moment, I can`t recall a freely available video editor with this feature but if I do I will amend this response.