The bus drivers' initial indifference to the volume can best be understood because the driver had adapted to the volume of the music
Step-by-step explanation:
The term adaptation refers to the meaning of being adjusted to something or something that we are used to it regularly. There won't be any difference to the environment that we exists because the things that we do regularly and we will be adapted to that.
But, when someone new to our environment feels some difference and say their suggestions we will be noticing the difference.In the example given, the driver hears the music with the sound that he is used to it or he hears in a regular manner. But, when the passenger told him about the sound, he realized that it was too loud. Hence, the driver best understood that he had adapted to the volume of the music.