Translation:Turn the music volume down a little so it won't disturb the neighbors.
因果关系 (because..so...) is sometimes expressed as in this sentence, without conjunctions; but more importantly, English doesn't allow strings of grammatically unconnected clauses like Chinese does, so "so" has been added here to make the English correct. (I'll admit I was slightly baffled myself when I first saw this sentence, not sure whether Duo wanted a subordinate phrase or just a semicolon!)
As to the lack of verb, Chinese predicates do not necessarily require one, but the wording given does sound slightly awkward. I would have gone with a "把"字句 like "请把音乐放小一点". Your "请降低音量" is acceptable as well.
I translated it as: "The music volume is lower, it will not disturb the neighbor."
There is nothing in the Chinese sentence to indicate a request or command is being given. It could just as likely be an explanation for why someone just turned the volume down or why they choose to play low music.
"The music volume is lower, it will not disturb the neighbor."
"The music volume is a bit low, so it will not disturb the neighbor."
Both of those should be accepted.