hear (something) in the background

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.

a siren

Sirens are devices that make a loud noise to warn people of things. Police cars, fire trucks, and ambulances have sirens. Military bases and fire stations might also have sirens.

The fire alarms that people have in their homes are usually not called "sirens". A siren has to have a sound similar to a police truck or ambulance.

some kind of (something)

The phrase "some kind of ___" can be used when you don't know exactly what you're talking about or you don't want to specify:

What's this? Some kind of stew?

I hear some kind of siren in the background.