I seem to recall reading somewhere that 'everything' is a thermometer, on the basis that many things behave differently at different temperatures.

You can also use an LED as a light sensor.

and I also came across a YT vid of a console that used a piezo electric speaker for motion sensing.

I wonder if you could use a track pad to pick up sound.

Yeah, but they aren't an input device with an amp wired in the right direction and an A/D converter to read it out.

If there is a discrete PA in the speaker path, then not. But I would not be that surprised if there is a single chip codec + PA combination that can conect an internal ADC to pins that are primarily meant as PA outputs of the integrated PA.