How a signal is transfered with only one connection in a simple circuit
You probably fried the DAC because the microphone input of a laptop provides a dc voltage source to properly bias the microphone.

The circuit, laptop side, might look something like this:







simulate this circuit &ndash; Schematic created using CircuitLab

mic_in is connected to the laptop sound card, jack_in is where you hooked the DAC, Vdd is some 5V, R is probably a couple of k\$\Omega\$ and C is in the 1uF ballpark.

Your DAC pin is an output. Outputs usually do not like being forced by an external voltage supply. But they are protected, and a couple of ks in series with it have limited the current to some mA, which is probably fine.

But you also hooked an earplug to it. And, while being an output, a DAC is usually not capable of directly driving such a big load, so probably the combination killed your DAC.