For example, Kenney's assets or something from OpenGameArt.com?

I'm thinking mainly about graphics, but question applies to sounds too.  Surely you don't expect us to create all our sound effects from scratch in three days?

Creating everything within three days is feasible. Look at the Ludum Dare compo, which has produced great games in two days, all original work. 

So no, you can't use non-scratch assets. However, scratch does have a large number of existing assets that you can use. 

If sound effects are your main concern, this may help you:

https://www.bfxr.net/