If you were to come up with one, I suspect you'd have a solid bug bounty waiting for you.

you just set the pc to not sleep on screen down? it is literally a feature

As far as I know the mic is still shut off when the machine is set to clamshell mode. That's the point. You cannot use the mic when the lid is closed. It's a hardware cut-off, you cannot configure it in software. Hence my comment about the bug bounty.

$5 USB mic?

If the point is to hide the recording that's not a great way. Especially when many corporate IT solutions monitor USB device connections.