I have a hardware accessing question. For a project, I was wondering if it would be possible to use the smartphone's speaker as a second microphone, similar to the idea that you can plug in a headset into your computer's audio jack and use it as a microphone.
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Sorry, as far as I am aware, there is no way to do this.
The speaker is highly ingrained into the hardware, and it would be highly difficult to get this to work - never mind that it could be impossible.
It could be possible, but it would require loads of hard work - including rewiring, and possibly some soldering - and your device may not survive.
Proceed at your own risk, but I would say that it isn't possible.