Little Lamb, who made thee?
Dost thou know who made thee?
Gave thee life, and bid thee feed
By the stream and o'er the mead;
Gave thee clothing of delight
Softest clothing, woolly, bright;
Gave thee such a tender voice
Making all the vales rejoice?
Little Lamb, who made thee?
Dost thou know who made thee?
Little Lamb, I'll tell thee
Blake is simply stating that the lamb’s bleating voice is so beautiful that the  personified  valleys themselves are happy to be in the lamb’s presence. 

The Lamb