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.

Can't speak for OP, but mine certianly doesn't. I know because I assembled it.

Mine doesn't have a speaker, either. It does have one of those wretched dingers that attach to the SPKR wires.

Sure that can't be used? I don't know what wretched dingers are.