Why can't they be? What's the physical mechanism behind it not working? Yelling into a speaker attached to an amplifier definitely produces electrical changes

Why can't they be? What's the physical mechanism behind it not working? Yelling into a speaker attached to an amplifier definitely produces electrical changes on the output transistors, changing the amount they're biased, etc. Does that produce no measurable input if the speaker is connected to a software-switchable input/output port? It doesn't have to be a large effect to be useful. Although at that point I think I'm more worried about the microphonic properties of ceramic capacitors in the signal path.