It's not known. Ask the average person that owns a smart TV if they think they're being listened in on. Or even what kinds of data they think their smart TV collects. I could bet you anything that for nearly all people, their answers would be "I never thought about that", "I didn't know they collected data" or "I thought it was only what I watched".

It's highly insidious - our society was reshaped to normalize data collection like this by adtech. At the same time, these invasive technologies were quietly integrated into existing devices people assumed they knew. TVs look like the same black rectangles they were 15 years ago. Cars today are similar to cars from the early 2010s. The average person doesn't understand how adding internet connectivity to some device exposes them to this until it's explained to them. To them, the notion that they record everything sounds as cospiratorial as "your microwave is secretly recording you on camera". If technological limitations required TVs with mics to have a huge studio microphone protrude on top of it with a flashing red light, a lot more people would suddenly care.

When I was a kid we used vacuum tube radio both as a passive speaker and a microphone... connected to the speakerphone of a telephone as a very poor man walkie-talkie.

Using the speakers as microphone when there is no outgoing sounds is pretty trivial in that regard. I don't know how paranoid a person can be, though.

During the Soviet era, not everyone could afford to have a landline phone, but the system kept hunting down its dissidents one by one. If you are a person of interest (which I believe doesn’t apply to you), there are way more important signals to adversaries than your microphone.

It's important to choose your threat model. If your plan is to resist surveillance capitalism or just a random Bob and his grandmother snooping on you online, luckily, there are some effective methods. Otherwise, I would say it’s always a question of time, not 'if' or 'how.'