The speaker is generally hooked up to a pre-amp and that pre-amp to the DAC. The hardware would need to also have an ADC attached to the speaker for that work. So such an attack could only happen if sound-chipset/card integrated into the motherboard also has an ADC sitting on the same analog line as the speaker output.
I have only ever seen the hybrid mic/headphone ports on laptops and not desktop motherboards. They may exist, but I have yet to see one. Although I guess what matter is how the ports are actually wired.
