In the general introduction to Before the Muses: An Anthology of Akkadian Literature (third edition, CDL Press, 2005) Benjamin R. Foster discusses the "Akkadian literary languages":
The Babylonians believed that literature required a special idiom, using grammar, vocabulary, syntax, sound, and even spelling removed from common usage. The most refined literary style of this period is known as the "hymnic-epic dialect". (...) In the Mature and Late periods of Akkadian literature, (...), another literary style known as "Standard Babylonian" was used.
However, Foster's introduction does not give examples that clarify how the hymnic-epic style and Standard Babylonian differed from each other. What exactly were the differences from the point of view of grammar, vocabulary, syntax and sound?
