Sounds are an integral part of a scene, helping the reader to determine whether the main character is alone, with a few people, or in a crowd.
The character can hear the sounds of laughter or arguing... Feet shuffling when they should be alone in the house... Steady, heavy footsteps on stairs... The sound of a refrigerator kicking on... Or the overwhelming sound of silence when electricity goes dead...
They can hear macaws or monkeys, conjuring immediately the image of a jungle... Or of seagulls and dolphins laughing, transporting the reader instantly to the ocean... The heavy-handed sound of a truck blasting its horn or a train whistle splitting the darkness...
It can be the sound of phones ringing and people talking... of music playing in the background... The sound of something crashing to the floor, but you don't know what...
Sound can draw you in or push you away. It can evoke feelings of security or terror. It can raise the red flag that something isn't right. Or it can lull you into complacency...
It can be the sound of surf hitting the sides of a boat, rolling in at intervals so unevenly spaced that it keeps you on edge...
The sound of the wind in a growing fury as a hurricane nears...
It can be the sound of a lover screaming for help in the darkness, their voice bouncing off the wind and the rain so you don't know from which direction it came...
The sudden, violent sound of thunder...
Sound rounds out a scene like no other sense can do.