But if they're not rendered, what about their sound effects like walking, or something like their bullets?

This is actually an issue in War Thunder, where if the server thinks you shouldn't be able to see a tank, it won't render it, but this also causes it fairly frequently to not play noises from the tank like the engine or shots, and to not render projectiles from them either. So a teammate can die right next to you and you won't know how because the shot wasn't rendered on your screen even though you were looking in the direction of the enemy when they fired it. Or a tank with an engine louder than a semi truck will sneak up and kill you because the game simply decided that you shouldn't be able to hear them.