From their sweet feeding in the fruit; the droning of the bees and flies, rises gradual as a lute " mean

Within these lines we see simile/metaphor, which is a literary technique of making a comparison between two things that are generally not considered similar in an effort to give readers a better understanding of the topic being discussed. In the lines we can see that the sound bees and flies make is being compared to the sound a lute (which is a type of guitar) in order to give readers who might be familiar with the sound of the lute (and unfamiliar with the sounds of nature) a basis for comparison in the understanding of how the sound bees and flies sounded. As such, what these lines are saying is that the sound flies and bees made while they ate fruit sounded like the music made by a lute.