Reflection.
Sound waves are reflected from the surfaces like light waves, obeying laws of reflection; the angle of incident is equal to the angle of reflection. Echos are examples of reflection of sound. This can be observed in a simple experiment. A source of sound such as a transmitter sends sound at a certain angle to the normal of a wall and a reflected sound in heard at another side of the normal. A higher sound is received from reflection only at the same angle of incidence. The regular reflection occurs from a surface if the whole part of an incident wave front is reflected uniformly. Since the sound waves in speech have wavelength of various meters, they are reflected from rough surfaces such as cliffs, walls etc. In a hall, sound reaches to a listener through many paths after reflection from ceiling and walls. Reverberation occurs when the sound produced at one instant by the source persists at the listener for some time afterward due to multiple reflections. If the reverberation time is too short due to much reflective surface, the hall sounds dead and if it is too long due to much absorbent, the hall sound dull that produces confusion. For a speech in a room, the acceptable reverberation time is about a half a second, while in music, it is between one and two seconds. In a design of an auditorium or concert hall, a balance must be archived between reverberation and absorption. The study of sound properties is called acoustics.
Refraction
Sound waves bend when the parts of the wave front travel at different speeds. This occurs in uneven winds or when sound is travelling through air of uneven temperatures. On a warm day, the air near the ground becomes warmer than the rest of the air, and the speed of sound near the ground increases. So, sound waves then tend to bend away from the ground, resulting in sound that does not seem to travel well. Due to the same effect, sounds are easier to hear at night than during the daytime. Different speeds of sound produce refraction.
Due to a similar reason, a distant observer hears a sound from a source more easily when the wind is blowing towards him than the wind blowing away from him. When the wind is blowing towards him, the bottom of the wave fronts moves slowly than the upper part and so, the waves bend towards the round the observer. Therefore, the observer hears the sound waves clearly and easily. However, in case of the wind blowing in opposite to the direction of speed of the waves, the wave fronts turns upwards from the ground and observer, and the sound is heard faintly due to decrease in intensity. That's why we hear thunder when lightening is reasonably close, but we often fail to hear the thunder for distance lightening.
Diffraction
Diffraction is the phenomenon of spreading of waves around the corners of an obstacle or apertures. It is a wave phenomenon and occurs if the wavelength of the wave is of the same order as the dimension of the diffracting obstacle. Due to the diffraction of sound through the doors and windows, sound is heard inside a room from an outside source. This is because the wavelength of sound wave is nearly equal to the dimension of doors and windows.
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