If you think that the information transferred by human speech consists only of the words spoken, you have a rather shallow view of language. You're also processing information taken from pace, pitch, timbre, intonation, volume, timing and direction (among other things). The written word is to speech what choreographic notation is to dance - it's enough to give you the gist, but you wouldn't mistake it for the real thing.

You're arguing a claim I didn't make.

Each beat is probably better considered a symbol, so by your analysis the human audio channel is around 20 baud not 20 bits/s. The symbol size obviously has a large impact on the information bit rate.

We can estimate the size of a symbol though: 13b for primary tones(20khz range at about 3Hz resolution) + 4b for rising/falling/etc tonal patterns + 7b for volume (1dB detection over 120db range) + (42 phonemes in english so round up to 256 for convenience) 8b for phoneme patterns + 2 ears preprocessed for 2d directions is maybe 4b direction and 4b distance gives a total of around 40b per symbol.

So with that wild approximation, it's at 800 bps, or 8.64 MB / 24h

A fair argument, and yes, that expands the channel somewhat.

(My first posts on seeing this concept, elsewhere, had confused bits and bytes, so I ended up with 1.7 MB rather than 0.2 MB.)

But the point remains that that what we're discussing is almost certainly a maximum upper bound on human conscious information processing capacity. And that the values are ... pretty limited, relative to the amounts of information our machines can fling at us.

8.6 MB/day means that on average we're looking at 2/3 of that for waking hours or 5.7 MB. If you spend half your waking hours at work, that's about 2.9 MB of information acquired per day at work, max, over audio track.

And that's the tail-end of a 3.5 billion year R&D effort.