When an audio source is added the audio will automatically play when the level loads.
You can find royalty free music at freemusicarchive.org. Make sure that you consider legal and ethical issues such as copyright and intellectual property when developing your outcome.
Open a scene
Add an Audio Source gameobject from GameObject > Audio > Audio Source
Drag the file from your file explorer into your project assets (make sure to use a sensible folder)
Drag the audio clip onto the Audio Source component in the inspector.
You’ve now added audio to a scene.
Adjust the settings as required.
To start playing when the scene loads make sure that Play On Awake is selected.