This is the 551st most frequent Japanese word.
高く
"高く" means "high" or "tall" in English.
Here, '高く' functions adverbially to describe the manner in which the subject raises his voice, emphasizing the action of raising the pitch or volume.
彼は声を高くして抗議しました。
He raised his voice to protest.
In this instance, '高く' is utilized to describe the pitch of the sound, modifying the verb '響いていました' with an emphasis on the description of how the sound is perceived.
その音は高く響いていました。
The sound reverberated at a high pitch.
In this sentence, '高く' is used as an adverb modifying the verb '難しい' in the phrase '登るのが難しい', describing the extent or degree of the height of the mountain.