Most likely it's the type of cable you're using. The phone either has a special pin-out for the AUX port or the cable is defective. The behavior you're experiencing is pretty common when the phone 'thinks' the cable you connected is for a headset but the pinout is incorrect or has a false contact, thus thinking you just plugged in either a headphone or, in your case, a microphone and is using the 'call' speaker to route audio. Have you tried it with a different cable and/or car/unit with AUX input (it may be the car unit, too, if the port is damaged or the cable doesn't really go all the way in)? Does it happen with headphones?
ncG1vNJzZmien6fCrr%2BNmqWdqp%2BesaSxza2pmqRemLyue9OhqZ6ZlKh8qbHLqWSmraOesG68y5qwrGWknb%2BwwcahZKaxXaW1sLrErGStp6BiwLGxwKScq2WRo7Fur8CrZKyolZa4pr7SZq6hnZ5isLC6zZ6arZ2UYsGwecCur2dpYGqEc4CQaA%3D%3D