I recently read about ad-sources correlating smartphones with desktops via transmitting ultrasonic tones that another device can listen for. I can't find the article now but it was mentioned on HN a few days ago...perhaps in a comment.
That's pretty vague. PC? Mac? Chromebook? OS version? Browser version?<p>Could be a browser extension; FB app you gave access to; malware; any number of reasons a website could access audio devices on your machine.