At the conclusion of the speech of the "still voice", the speaker voices resume for the concluding section, state the main thrust of his advice.
The conclusion of the poem 'Thanatopsis' by William Cullen Bryant has some consoling things to say about death. However, it has to be said that the poet wrote the piece when he was very young! Such youthful types tend to have a much rosier outlook on Death than older people. At the time of writing, Cullen Bryant had been given a mistaken diagnosis and his family were expecting him to die, so the comforting words may have been written partly with them in mind. He did not die of course , but the poem remains. The main thrust of his advice is that we should accept death without fear as part of the natural cycle of Life, as the earth that we will become feeds the roots of a beautiful tree. Life is an ecological cycle and we are part of that.
I am assuming that the concluding section is the part from line 73 to the end of the poem.
The main thrust of those lines is that people should not fear death. They should not be dragged to the grave kicking and screaming (figuratively, of course, seeing as how they're dead). Instead, people should face death as if they were just going to go and have a restful nap. They should expect that when they lie down for that nap they will have pleasant dreams.
This is in keeping with the main idea of most of the poem, which is that people should not fear death so much.