“I well remember the time this year when I first heard the dream of the toads. I was laying out house-lots on Little River in Haverhill. We had had some raw, cold and wet weather. But this day was remarkably warm and pleasant, and I had thrown off my outside coat. I was going home to dinner, past a shallow pool, which was green with springing grass, and where a new house was about being erected, when it occurred to me that I heard the dream of the toad. It rang through and filled all the air, though I had not heard it once. And I turned my companion’s attention to it, but he did not appear to perceive it as a new sound in the air. Loud and prevailing as it is, most men do not notice it at all. It is to them, perchance, a sort of simmering or seething of all nature. That afternoon the dream of the toads rang through the elms by Little River and affected the thoughts of men, though they were not conscious that they heard it.”
I’ve underlined the word in question. A misprint for drone? Guy Davenport asked the same question in a letter written on Aug. 29, 1997 (Guy Davenport and James Laughlin: Selected Letters, 2007):
“In Thoreau’s journal the other evening I found the strange phrase `the dreaming of the toads’—and was mystified until I tracked down an archaic meaning of dream meaning music or `a joyful noise.’ Perhaps the ancestor of drone. Now I’d like to know how so old a meaning survived for Thoreau to know it.”
That's a very old meaning from the Old English, says the OED. Most citations date from the thirteenth century and a few are older. This meaning, as a noun, is judged “obsolete": “The sound of a musical instrument or singing voice; music, minstrelsy, melody; singing, a song. Also: noise, din; clamour, lamentation; voice, speech.” So, too, the verb: “To make a joyful noise, rejoice; to sing or make music.”
We know Thoreau read Anglo-Saxon poetry at Harvard, though it seems not to have been of particular interest. The unlikely multiple meanings of dream would have amused Thoreau, however, as would the scriptural echo.