How do you make sound effects from different sources sound coherent together?

Sound is the one subject I've been procrastinating and now I'm finally at a point where I want to polish it. My "strategy" so far was just to download whatever sounded close enough on freesound, though of course it sounds terrible when you put a bunch of disjointed sounds in the same context. If we were talking about bought art assets I could change the color pallete, the pixel size, texture resolution, etc, all in an effort to make everything look coherent. What is the equivalent for sound?