From TechCrunch: "Google’s speech recognition technology has made massive strides, though, and while the service still makes its fair share of mistakes, it’s now quite good at making videos accessible to the deaf and hearing impaired".
According to Google: "YouTube can use speech recognition technology to automatically create captions for your videos. The wide variety of content on YouTube obviously makes it hard to caption everything, but it also offers the company with a wide range of training data. When the YouTube community reviews and edits these automatic captions, that information then flows back into the machine learning process to make the system more accurate. The service’s automatic captions currently support 10 languages."
TechCrunch: "Google says the service is getting closer and closer to human transcription error rates and that all the various improvements the company has made over the years have led to a 50 percent increase in accuracy... the service has now captioned one billion videos and people watch video with captions 15 million times a day."