That is a good question, but as I understand it (from reading only) emscripten does a few other things, such as convert opengl calls to webgl, as well as use sp

That is a good question, but as I understand it (from reading only) emscripten does a few other things, such as convert opengl calls to webgl, as well as use special integrated versions of SDL and GLFW so that native programs that use those libraries can be compiled directly. Both of those would include input and I think audio is dealt with in a similar way.