There are always a lot of reasons to see what we see on ads and suggestions without them having to listen to us. Try to do the test and talk about something completely random to you around your phone. Chances are you'll never get ads about it.
The algorithms are based on so many criterias and are so freaking good that it seems like the simplest answer is to listen to us. But with GPS, relationships, history, habits, emails/sms/messages, etc. it can be freaky how good the predictions can be. They are already "listening" in so many ways that are cheap to do, constant audio streaming is absolutely not cheap and not required.