Your speakers are a microphone ..

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.