Autobiography, p. 191:
By hearing him often, I came to distinguish easily between sermons newly composed, and those which he had often preached in the course of his travels. His delivery of the latter was so improved by frequent repetition, that every accent, every emphasis, every modulation of voice, was so perfectly well turned and well placed, that, without being interested in the subject, one could not help being pleased with the discourse; a pleasure of much the same kind with that received from an excellent piece of music.
Whitefield was famous for preaching outdoors to large crowds of hearers. His voice was remarkable for its ability to carry and many testified of its range.
Edward M. Panosian, "George Whitefield: The Awakener," in Mark Sidwell, ed., Faith of Our Fathers: Scenes from Church History, p. 146:
That voice has been occasion for considerable wonder in the two centuries since it was last heard on earth. When preaching out of doors—which had become for him more the rule than the exception—Whitefield would position himself so that his voice could be carried downwind to reach the greatest number of hearers. That number was often recorded as more than twenty-thousand in the journals of eyewitnesses. They testify also that Whitefield’s voice could be heard more than a mile away.
But Franklin is his most famous earwitness, and with his acute mental faculties, he devised a method of ascertaining just how far Whitefield's voice could carry.
Autobiography, pp. 190-191:
The last time I saw Mr. Whitefield was in London, when he consulted me about his Orphan House concern, and his purpose of appropriating it to the establishment of a college.
He had a loud and clear voice, and articulated his words so perfectly, that he might be heard and understood at a great distance; especially as his auditors observed the most perfect silence. he preached one evening from the top of the Court House steps, and on the west side of Second Street, which crosses it at right angles. Both streets were filled with his hearers to a considerable distance. Being among the hindmost in Market Street, I had the curiosity to learn how far he could be heard, by retiring backwards down the street towards the river; and I found his voice distinct till I came near Front Street, when some noise in that street obscured it. Imagining then a semicircle, of which my distance should be the radius, and that it was filled with auditors, to each of whom I allowed two square feet, I computed that he might well be heard by more than thirty thousand. This reconciled to me the newspaper accounts of his having preached to the twenty-five thousand people in the fields, and to the history of generals haranguing whole armies, of which I had sometimes doubted.