The "white speck whose sound ha[s] disturbed" Old Musoni's work is an airplane.
Explanation:
At the beginning of the short story "The Setting Sun and the Rolling World," the character Old Musoni is working on his land, tilling the soil. However, the sound of a "white speck" distracts him. He looks up and sees the white speck quickly disappearing over the rim of the sky. We can already assume it is something of great speed, which can also produce noise loud enough to be heard by the man and to distract him. As the story proceeds, the identity of the white speck is made even clearer when it is described as a "white metal bird." The white speck was an airplane, whose sound reached Old Musoni in time for him to see it disappearing over the horizon.
