In this case, the person is not using the word literally in its true meaning. By pretending that the statement is not exaggerated, the person stresses the fullness of the store. Literal language is language that means exactly what is said. Most of the time, we use literal language. The opposite of literal language is figurative language.
Figurative language is language that means more than what it says on the surface.
It usually gives us a feeling about its subject. She does not mean that a truck can actually sing. Rather, she is speaking figuratively. She is referring to road noises as music. By using the word song, and suggesting music, she brings joyful feelings to mind.