48 hours is super short, so  allowed to use pre-made assets is great,

but can we use some free assets to makes game looks cooler? like background arts, audios.

Yes you can. If it is free on a website such as the unity asset store (if you use unity) then you are legally allowed to use it. If there are any conditions such as putting the creators name in the description of your game, you will have to comply with those.