For prototyping I usually use Kenney assets (from kenney.nl). Theres lots of assets for many types of games which lets you rapidly prototype and iterate without worrying about making assets first. For sounds you can use BFXR and just play around. Sometimes playing around with sounds can even lead to new feature ideas :)
While most of the stuff is one off assets that do not fit together there are also some nice sets by some creators such as Kenney or Emcee Flesher
Also the Liberated Pixel Cup (LPC) stuff is pretty nice.
Mind you I mostly just look for 2D assets.
https://opengameart.org/users/kenney
https://opengameart.org/users/emcee-flesher
https://opengameart.org/content/lpc-collection