Final answer:
The word "REALLY" in the sentence emphasizes that the music was exceptionally loud, indicating a high amplitude and intensity of sound.
Step-by-step explanation:
The nuance conveyed by the bolded word "REALLY" in the sentence "The music was REALLY loud in the apartment down the hall" suggests an intensity or a degree that is beyond usual. It signifies that the music was not just loud, but very loud to the point of being noteworthy or excessive. This word is an adverb that is used to emphasize the adjective that follows, which in this case is "loud". In terms of sound dynamics, this would likely mean that the sound has a high amplitude, leading to a higher intensity, and therefore, greater loudness.
The term "REALLY" elevates the statement, implying that the loudness of the music is at a level that could cause disturbance or be clearly heard at a significant distance.