Anything and everything.
Starting a VirtualBox virtual machine results in a warning popup:
This is because Windows/the audio driver has disconnected the devices due to nothing being plugged in to the relevant ports. Modern audio hardware has a feature called jack sensing, and can disable the respective devices when there’s nothing to receive input from or send input to.
Plugging my headset/mic combo into the front-panel audio connectors results in the microphone recording source being enabled.
If you want these sources to be available to your VM, ensure they’re plugged in before starting it.