I'm an English-native speaker and I have no problems understanding Sal during the videos. For me, the closed-captioning feature is distracting. It's a good, necessary, and important feature for many users, but it's not something I need.
Each time I play a video, I'm required to press the "cc" icon to turn off closed-captioning. This gets repetitive. Could the Khan Academy website instead remember the "cc" preferences for each user between videos? That way, people would only need to click the "cc" icon once to change their preferences. This wouldn't require a separate click for each individual video.