Here are some links to go to get free art assets to use in your game:

• Kenney Game Assets — All game assets available on the Kenney website, free
  
• https://assetstore.unity.com/ - The Unity Assets store contains tons of free 2d and 3d assets
• https://opengameart.org/ - mainly 2d art assets
• https://www.gameart2d.com/freebies.html

For sound and music:

• https://assetstore.unity.com/ 
• https://www.zapsplat.com/sound-effect-category/game-sounds/
• https://www.freesound.org/browse/
• http://kenney.nl/assets?q=audio
• https://opengameart.org/