"Turn the music volume down so it won't disturb the neighbors."
"music volume" is translated in another sample sentence as "音樂的聲音“. Is it necessary to use 的 in this expression or is this optional?
You can say it without 的 but behind the scenes that changes the syntax quite dramatically. Basically 音乐 is no longer a qualifier to 声音 but the sentence topic: "as for the music, the volume is too high."
Both are valid translations of the English sentence though.
“不要 影响邻居” is better translation for the second part of the sentence.
“音乐声音小一点儿，(就)不会影响邻居” sounds like “*If you turn down the music volume a bit, then it won't disturb the neighbors”.
This exercise is still missing support for the "一點" (non‐erhua) answer variant: "音樂聲音小一點，不會影響鄰居。"