When you're listening to a telephone call, a radio show, or watching a video, sounds that are not close to the microphone are "in the background":
Listen - you can hear Rachel saying something in the background.
You can also see things in the background in a photo, a painting, or a video.
See that mountain in the background? That's Mt. Mitchell.
Sounds or sights that you see in person (not on a phone call, in a movie, in a photo, etc.) are usually not described as being "in the background". Instead, you describe them as being "in the distance", "in the other room", and so on:
I hear some people talking outside.