Yes but intruding into private audio calls is something users would be heavily suspicious of. It's not hard to imagine a non technical users thinking "OMG they're showing me ads based on my calls". Lots of people already think this.

Also its one thing to show an expensive ad on a screen that the user is actively looking at (that might actually be effective), and another to show an expensive ad on a screen that 95% of users are not looking at.