If your create a new Google Account, you would saw a new symbol beside the word verification captcha form.
Look out for this icon
If you hover the mouse over the icon, it says “Listed and type the numbers you hear“. And clicking on the icon, a voice clearly speaks out the voice captcha.
Captcha (an acronym for “completely automated public Turing test to tell computers and humans apart”) is a type of challenge-response test used in computing to determine whether or not the user is human. It requires the user to type the letters of a distorted and/or obscured sequence of letters or digits that appears on the screen and prevent prevent bots from using various types of computing services like taking part in online polls, registering for free email accounts etc.
But Captchas based on reading text or other visual-perception tasks prevent visually impaired users from using these service. This Google’s step to provide a voice captcha / audio captcha provides accesibility to websites and will lead to all captcha developers incorporating such features in their services.