Who, through long days of labor, And nights devoid of ease,
In this line the speaker is still talking about the "humbler poet" he brought up in line 25.
This guy, this down-to-earth writer, worked long days, and had hard nights where he was uncomfortable and unhappy ("devoid of ease").
This kind of poet wasn't some kind of faraway "bard." The speaker imagines this poet having the same kind of hard long days and nights that he's been having.
Still heard in his soul the music Of wonderful melodies.
The important thing about this poet, though, is that, even when things are hard, he can still hear music in his soul. In other words, instead of reminding us of how hard life is, he brings us "wonderful melodies" that soothe our souls.