neigh, whinny
嘶 centers on the high-pitched vocal sound a horse makes: neighing or whinnying.
嘶 combines 口 (mouth) with 斯, which likely contributes the sound. The character expresses the idea of a vocal sound made with the mouth, specifically a horse's neigh.
The mouth radical 口 on the left and the right side 斯 together suggest a loud, sharp sound coming from a horse's mouth: a neigh or whinny.
For セイ, imagine a horse neighing a loud 'say!' as if trying to speak: say -> セイ, and the horse's mouth 口 is wide open.