koto
筝 refers specifically to a traditional Japanese koto, a long zither-like stringed instrument.
筝 combines 竹 (bamboo) with 争, which likely contributes the sound. The character originally referred to a bamboo zither, and the modern form is a simplified version of 箏.
The top 竹 suggests bamboo, and the bottom 争 looks like strings stretched over a frame. Picture a koto made of bamboo with strings pulled tight.
For ソウ, imagine the soulful sound of a koto: soul -> ソウ, and the bamboo instrument sings.
guzheng (type of ancient Chinese zither)