I'm playing around with an oscilloscope and my laptop's headphone jack. I get the tip/ring/ground and measure between them. I also measure between them and an external reference ground. I notice I can see when the battery is charging. That is, if the battery is not in the laptop but the AC is in, then I insert the battery, it runs through a few seconds of a test, and then if the battery is not full, it goes to a certain waveform – if it is full, it goes to another "quieter" waveform. This is best done when the laptop is turned off otherwise the waveform is quite noisy.
I'd like to be able to understand more about what is going on. Might there be any other data present in the signal on the headphone jack if I am not putting out an audio waveform? ie. by virtue of the circuit passing through some of the data processing elements of the motherboard, when the computer is powered on? There is a lot of noise in the signal but maybe it is possible to clean it up some.
I thought this was interesting as it is a similar idea: http://cs.tau.ac.il/~tromer/acoustic/ (Here the noise comes from a bank of 1500µF capacitors).