Install Little Snitch for Mac and reboot. You can use the filter to limit external communications to just those that you explicitly authorize. If it's an ongoing thing, just disconnect for a while to see if it persists.
If you're really worried about it and don't want to monitor network connections, you could always block the microphone. On my MacBook, I used a few layers of electrical tape to cover the microphone (didn't fall off until a while after I stopped being paranoid). To find the microphone(s) on your machine, you can open QuickTime Player, click
File > New Audio Recording, turn the volume up, and tap around the chassis. Use headphones to eliminate reverb loops while testing.
Really doubt an eavesdropper would use a two-way stream.