Combined radical meanings are rarely evident at first sight.
Howell for example gives the following information:
音 (9) オン；イン；おと；ね
The relevant seal inscription form shows 言# words + a line in the mouth element at the bottom →
indistinct utterance contained in (not emanating as speech from) the mouth
(compare 吟) → sound; noise; tone → news/word of; ON reading of a Han/Chinese character (← read a character by its Sino-Japanese sound; the contrast is to 訓, where the idea is to grasp the signification of a character).
暗 (13) アン；くら（い）
As per 音# (indistinct) + 日 sun/day → scant light → dark; dim → secretly; hidden;
gloomy; ignorant.
To put it more in layman's terms, it's a good idea to drop the idea that the meaning of a kanji simply comes from the combined meanings of all its elements. That works for a small percentage of kanji, but most kanji are not formed in that kind of simplistic way.
暗 is what's called a phonosemantic kanji, where one half contributes the sound (as it sounded in ancient Chinese) and one half contributes the broad category of meaning.
It's easy to see how the concept of "darkness" broadly relates to 日.
音 does mean "sound" on its own, but it's not being used for its meaning here. It's being used for how it's pronounced.
I'm not sure where your mention of "music" came from. Perhaps you confused it with the meaning of "sound"?
But anyway, it's basically "the concept related to 日 that sounds like 音". This way of constructing kanji is the biggest category. So, resist the urge to just assume all the parts are contributing an obvious meaning to the overall meaning.
radicals => meaning
how does "day", "music", and "stand" can form something like "darkness"?