[dessant]

There are free speech recognition services, such as Wit.ai, and a number of others that have generous free tiers. Also, it's likely that spammers will not bother finding a free public API, and will just use the endpoints of Google Voice Search, or any other similar service from other companies.

The unCaptcha paper and the team's research is very much relevant, because it informs the public about the effectiveness of these security systems, and it helps website admins consider these threats and possibly adapt to them.