。
She spoke in a gentle tone.
Literal
She [topic-は] soft tone [with-で] spoke.
柔らかな口調で (in a soft tone) uses the [quality + で] manner-marking pattern, where で indicates the manner in which something is done. 柔らかな is the pre-nominal form of 柔らかい (soft/gentle), and 口調 (くちょう) means tone of voice or manner of speaking — distinct from plain 声 (voice), 口調 covers the overall verbal demeanour. The polite past 話しました gives this a neutral, conversational register — someone describing how she spoke to a third party.