The creature hears the old man play a guitar.
The creature is surprised to hear music for the first time, and it affects him deeply. When watching the inhabitants of the cottage one day, the creature sees the old man play a guitar.
[She] took something out of a drawer, which employed her hands, and she sat down beside the old man, who, taking up an instrument, began to play, and to produce sounds sweeter than the voice of the thrush or the nightingale. It was a lovely sight, even to me, poor wretch! who had never beheld aught beautiful before. (Ch. 11)
You have to remember that the creature has had very little human interaction. He has experienced little of beauty, but he feels deeply and learns very quickly. He is much affected by this music. It touches his soul. The creature notices that the old man’s “sweet mournful air” makes the girl cry. The interaction between the two of them is more than the creature can handle, since he has had no human love and was rejected by his creator.
He raised her, and smiled with such kindness and affection that I felt sensations of a peculiar and overpowering nature: they were a mixture of pain and pleasure, such as I had never before experienced, either from hunger or cold, warmth or food; and I withdrew from the window, unable to bear these emotions. (Ch. 11)
Later that night, the old man plays the “divine sounds” of his instrument again, but another man reads. The creature hears words read aloud for the first time. Through watching this family, he is seeing a microcosm of human life and experiencing it. He notices that they are not always happy.
“It was on one of these days, when my cottagers periodically rested from labour—the old man played on his guitar, and the children listened to him—that I observed the countenance of Felix was melancholy beyond expression; he sighed frequently; and once his father paused in his music, and I conjectured by his manner that he inquired the cause of his son's sorrow. (Ch. 13)
Of course, he cannot remain an observer forever. The creature eventually enters their world as a participant. Since the old man is blind, he does not see how hideous the creature is. He says he is “a traveller in want of a little rest” (Ch. 15). When Felix sees him, he is horrified and angry, and chases him away. His life with the cottagers is over.
The little cottage life that the creature has, briefly, is a fantasy. The creature is always on the outside looking in. He is able to look at the cottagers and mimic them, and learn from them, but he is never assimilated into their world. Still, seeing them gives him pleasure. He becomes more human from this experience, even though he experiences cruelty from it that hardens his resolve and makes him more into an outsider and a monster, and sets him more on a course of destruction against Victor.