In Robert Gallager's lecture notes for 6.450 Principles of Digital Communications I, Exercise 2.1 asks an interesting conceptual question:
Voice waveforms could be converted to binary data by sampling at 8000 times per second and quantizing to 8 bits per sample, yielding 64kb/s. [...] Modern speech coders can yield telephone-quality speech at 6-16 kb/s. If your objective were simply to reproduce the words in speech recognizably without concern for speaker recognition, intonation, etc., make an estimate of how many kb/s would be required. [...] (Note: There is clearly no “correct answer” here; the question is too vague for that. The point of the question is to get used to questioning objectives and approaches.)
My first instinct was to look up number of words used today in English language (~170,000), calculate how many bits would be required to brutely encode each word (~18), and look up, on average, how fast we speak in terms of words per second (~2), and come up with 36 bits per second as my crude answer. I'm not sure if not concerning with the speech waveform itself, and all the signal processing that would entail, is a reasonable way of thinking about the problem. I'm here for feedback and other ways of thinking about this problem.
(This is not a homework assignment. I was studying the notes myself and I'm curious.)