。
She spoke to me in a low voice.
Literal
She [topic-は] me [to-に] low-voice [in-で] spoke.
小声 (こごえ) literally 'small voice' — the native Japanese way to say 'in a low voice, hushed tones, quietly.' Contrasts with 大声 ('loud voice'). The で here is the manner/means particle: '[in/with] a low voice.' This particle also appears in expressions like 日本語で話す ('speak in Japanese'), ペンで書く ('write with a pen'), 電車で行く ('go by train') — same particle, different shades of 'means.'