Yes, as long as one still needs to look at the screen there is still scope for ads. On-screen ads, if they are small enough, aren't annoying and sometimes they are useful too. There will be some interesting dynamics regarding how ad money is distributed among all the api backends.
But if voice response is used a lot, then ads(in the audio) will be sufficiently annoying for many people to switch to paying the equivalent small amount every month for ad-free services (which will have to be be distributed to the backend providers using micropayments). If this kind of service works out, then the mechanism exists for getting rid of the on-screen ads too, whether or not people actually use it. I wonder how much a the ad-value for a typical user is every month. A very rough calculation 30 billion in google's revenue/(say 200 million users * 12 months) gives 12.5 dollars/month. I think this should be an upper bound on the average, and that there will be a large variance - users who consume drugs will have larger value for advertisers, for instance.