>That's pretty fucking clear to me.
Said nobody who understands law, language, or politics. It's absolutely fucking clear to everyone who reads with discrimination what this means. It means they are using your microphone to vacuum up your information. That they added the qualifier "..to inform ads or to change what you see in the News Feed" gives them the verbal and legal wiggle room they need to do absolutely anything they want with your information. For example, they could use your microphone to monitor, record and quantify everything they hear to "adjust a user profile" that they keep on all users. In that case, they could simply claim that the ads you see and the news feed you get are based on your user profile.
>How could they respond to remove all doubt they're listening?
They could very simply. "We do not listen to your microphone or record content, data, or metadata from your microphone in any circumstance". Without qualifiers of any kind.