A yogin, at the start of his practice, concentrates in the inner side of right ear from inside and, hears many sounds proceeding from those like an ocean and clouds( It is said that this starts from sounds of crickets and other familiar sounds and at higher levels it is Sounds of flute, cymbals and Cloud thunder). These filter out, and over time, he can hear more subtle sounds such as single note of musical instruments at his will, and get absorbed in whichever sound note it wants to, asserts the text. This focus on sound notes help the yogi destroy distractions from other senses and fluctuations of his mind, just like a bee focussed on honey does not care about the odour that surrounds it. Such a yogi does not care about fame or disgrace from others, he does not feel heat or cold, neither joy nor sorrow, he is lost within, in his self, in Brahman-Pranava (Om).
Source: Nadabindu Upanishad – Wikipedia