What if a wave reaches a type of

matter it cannot pass through? Does

it just stop moving? If not, where does

it go?

If it hit the matter that it cannot pass through ... according to the velocity of wave ... it might get reflected back ... we can find sound waves reflecting back in closed rooms ... because of the presence of the matter through which it cannot pass ... and ... it's because of this tht ... echo is observed

It is bounced back. Like echoing, it's basically the sound wave hitting the matter it can't go through and bounce back.