It's surprisingly easy to use adtech without voice and make a connection to serve a targeted ad. Had a friend ask me about what I was drinking. They were on my guest wifi network. They searched for it. Next day, I'm getting ads because of geoIP pinned my IP address as having an interest.
Also had someone that lives off the grid with no active network or devices watch a DVD of a movie and the entirety of their Internet connectivity was two cell phones in the room. They started seeing things related to the movie. They're older and not constantly on their phones. The phones just sit somewhere in the room.
Had a discussion with some tech friends a few years back and remarked that keeping awake to do this would take a lot of power. The EE mentioned running audio recording would take basically nothing. I expanded from there, the device uploads audio for off-phone translation to text, or queues batch jobs to process locally when power is high enough or on charger. Etc.
It is 100% probable that code runs on phones and just ships off amalgamated text frequency charts or entire conversations and the user won't even notice the battery dent.
That being said, I can't find even in the greediest capitalist money-claw that the person giving a go would not think, "well, I can't trust my own device anymore..." and maybe go: "yeah, I shouldn't do this." Maybe I'm too optimistic though.
And the health apps know when you're sleeping, they know your heartrate throughout the day, your o2 sats. They can take all this mortality risk data to factor in things, advertise drugs to you, advertise foods they know you'll eat even though it's bad, manipulate how your insurance pays out for your next treatment because it would have been preventable if you hadn't eaten those donuts. The phone manufacturers know you run apps, how long, what you do (yes, even Apple, especially Apple, they hide behind "privacy" so you feel ok with what they do to you) what web pages you open, how long you view them.
They could biometrically paint a picture of your day, your movement, there's an entire profile of data available on many humans. I wouldn't be surprised if they aren't already tying heart rate data to viewership of media and advertising.