Although Sound velocity is maximum in solid then why two people in two sides of the wall are not able to hear each other?

Answer 1

Although sound does travel faster through solid media than gaseous, many walls are filled with materials or structures designed to absorb sound energy instead of transmitting it. In addition to this, sound gives up some energy as heat whenever it travels from one medium to another, so going from the air to the wall's surface to the inside of the wall to the wall's other surface to the air on the other side does a great deal to dampen the sound. The combination of these effects is often enough to lower the intensity of the sound past the threshold for human hearing.