Answer:
As the boy walked down the street, he sang a song about living happily ever after.
Step-by-step explanation:
This is singular. "Was walking" is correct, but the second part is not. "The boy walked down the street," is correct.
The term "sang" is more commonly used than "sung." “Sung” is a past participle. It's used only after a helping verb: “She has sung the anthem." "Sang" is grammatically correct in this context.
Hope this helps! :)