Only mammals have an auricle or pinna, which is the flap outside the external ear opening, along with three middle ear bones "behind" the eardrum (hammer, anvil, stirrup).
But birds and some other reptiles (zoologists now classify birds with the reptiles--bit of trivia--last of the dinosaurs) do have an opening in the side of the head with the eardrum right below it, and they have the "stirrup bone" in the middle ear to transmit sound. As an aside, the other two middle ear ossicle bones that mammals have are derived from ancestral jawbones that were repurposed in mammals but still present in birds/reptiles. It's a beautifully documented evolutionary transition in the fossil record.
I wouldn't say mammals hear better than birds and lizards, though having an external pinna makes it easy to localize sounds. Maybe birds compensate for the lack of an ear flap by having such flexible necks and constantly moving their heads around. Birds clearly have excellent hearing.
But snakes and turtles/tortoises lack even the opening to the outside with the eardrum. They pick up low frequency sounds and vibrations through their jawbones, but they don't "hear" high pitched sounds and have pretty limited airborne sound range in general. With snakes, the thought is that they evolved from an ancestor that became a burrower, so losing limbs and eyelids and external ear openings made sense, though many (most) snakes today spend most of their time above ground.
I'm not sure there's a strong hypothesis why turtles and tortoises lost their ear opening. They branched off from the other non amphibian vertebrates early on and are just different in a lot of ways besides their shells. It's certainly the case that they live close to the ground and don't climb or do other things where hearing airborne sounds from far away is super helpful, and early turtles seem to have been an aquatic group.
If real zoo tortoises get stressed by lots of guests around their enclosures, it may be because they are picking up on their footsteps and other lower frequency sounds, of course.