a koto
箏 refers specifically to a koto, the traditional Japanese stringed instrument. The meaning is narrow and concrete, centered on this particular musical instrument.
箏 combines 竹 (bamboo) with 爭, which likely contributes the sound. The bamboo radical points to the material used in making the instrument.
The top 竹 is bamboo, used to make the koto's body and bridges. The bottom 爭 looks like a hand plucking strings. Together they form the koto.
For ソウ, imagine the soulful sound of a koto: soul -> ソウ, as the bamboo strings resonate.