Simile is a type of figurative language that establishes a comparison between two different things so as to attribute to one the quality of the other. In that sense, simile and metaphor are the same. The difference between them is that simile relies on words such as "like" or "as" to make the comparison, while metaphor does not.
With that in mind, we can easily spot the simile in the lines:
"The droning of the bees and flies / Rises gradual as a lute."
The simile above is comparing the sound made by the insects to the sound of a musical instrument. The lute has a sound similar to the guitar, but higher and brighter. Perhaps the author of the lines wishes to imply that the sound made by the bees and the flies is as sweet and beautiful as the sound of a lute.
