Walking and talking are both instinctive, acquired behaviors, not learned knowledge. This is why you can't teach a child their first language or how to walk - they have to reach the developmental stage where this process will happen automatically, and teaching has little-to-no effect whatsoever on the acquisition process.

This is also why amnesiacs are always able to walk and talk (unless they've also specifically undergone damage to Broca's or Wernicke's area, or the motor cortex).

Also, even if humans somehow did "forget" how to talk, language is a human instinct, communication would resume almost immediately, and within a single generation new, fully-featured language would have developed. We can see this with Nicaraguan sign language, for example.

Also also, it would be very difficult to completely drive humanity into extinction. Even if that generation "forgot" how to walk and talk somehow (whatever that might mean), it's basically guaranteed that plenty of humans would survive across the surface of the earth to continue the species nonetheless.