A bunch of high-resolution photos under Creative Commons Zero ("do what you want") license are at Unsplash.
You can also check Flickr for images with the "Mods & Commercial Use Allowed" license option if you can give attribution to every asset, or "No Known Copyright Restrictions" license flag if you don't want the burden of keeping notes on sourcing/attribution.
Since these will mostly be photos not all of them will be suitable as backgrounds depending on your style, but since they're do-what-you-want or Modifications allowed, you can totally crop/filter/redraw/trace/whatever from them without any worries!
For audio you can look at Freesound.org (in search results, the little "person" icon means attribution required, versus a "0" icon which means no rights claimed.)
And then of course Archive.org has a bit of everything, which gives it a bit of a discoverability problem. But for something like music: if you can find a genre that fits your game, you'll probably have everything you need right there!