。
She turned down the radio.
Literal
[topic-は] radio [genitive-の] sound [object-を] small-made.
小さくする is the い-adjective 小さい ('small') in its adverbial form 小さく plus する ('to do, make') — together meaning 'to make small.' This ~くする pattern is the productive way to say 'make X [adjective]': 大きくする (enlarge), 暖かくする (warm up), 安くする (lower the price). Japanese uses 'sound + small/big' to express volume control, paralleling the conceptual frame of physical size for auditory intensity.