I don't know, but that sounds cute. I've seen dogs howl when their owners sing. It's like they're trying to sing along.
I'll never forget about the 3rd night Jake was home with us, and he howled. I was sitting on the couch with him, and he suddenly jumped up and stood with his body straight (on my lap) and howled! He must have heard something outside because then he was barking viciously and acting like he was my big protector.....all 1 lb. 10 oz. of him (at the time).
I have only heard him howl that one time.