、。
Shh, keep your voice down.
Literal
Shh, voice [subject-が] loud.
しーっ is an onomatopoeia for 'shh' — the sound of shushing someone. 声が大きい 'voice is loud' is the standard way to tell someone they're being too loud in Japanese. No imperative needed — the statement of fact functions as an indirect command. Ultra-short and universally natural.