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I wonder what the bandwidth limit would be if you took 44.1kHz audio transmission to the extreme. Something like 56k6 modems?

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Probably quite a bit higher: POTS was band-limited to something like 300-3000 Hz, which covers speech pretty well but not much else (hence the tinny hold music).

If this is true, then a brief 30s audio signal would yield more then a A4 photo capture of high density visual encoding, like microsoft hccb barcodes, interesting

Raw audio files are pretty massive.

A .wav file is 16 bits/sample x 44,100 samples/sec ≈ 700 kbps. Thirty seconds of that is about 2.5 Mb, which is a reasonable size for a photo. However, you wouldn't be able to send nearly that much data, since you'll be limited by various kinds of noise.

But wouldn't POTS imply a far less noisy environment once filtered?

It's a tradeoff.

You're now nyquest-limited to sending fewer symbols per second (baud), but you might be able to use a larger "vocabulary" of symbols.

This was one of the big changes in modem design. The first 300 baud modems used one 1 bit/symbol, but the V34 modems used a bigger symbol set that could send 6-10 bits/symbol (and also sent the symbols faster).

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