The Singing Heart – may not be the title, but that is how I remember it.   It was a hardcover I encountered as a child in the 70s.   It was heavily illustrated with black line drawings and was probably aimed at ages 9-12 perhaps.    In it a woman (maybe a good witch) dies, but her heart goes on singing.   Two kings wind up trying to get the singing heart; eventually they meet as beggars and become friends, having lost everything.   Pretty sure there is a wicked witch with a wicked dog.   In my mind the illustrations are similar to some in the Glass Harmonica, but I’m not sure about that.   I think one of the kings was named “Bagdamugus” or the like.

Thanks for any help!