Keep it down, will you? 
→  소리좀 낮춰줄래?

[예문]
A:Keep it down, will you? It's far too loud.
  소리좀 낮춰줄래? 너무 시끄러워.

B: Sorry, I'll use my headphone.
  미안해.해드폰을 써야겠다.