Up until the past week, I'd only heard Elmer bark on rare occasions. He's a very quiet dog. David taught him to "speak" one day while I was at the vet with Scooter. Elmer is very proud of his voice now! We've noticed that he usually only barks when he's sitting, and he's a little more shy around Scooter.
I was playing around with the dogs today and took a few videos. You'll notice that Scooter LOVES to speak, but he's not a very yappy dog luckily. He only does it when there are treats.