sound, voice, noise, tone, music
聲 centers on audible phenomena: sound, voice, noise, tone, and music. It encompasses both the physical sensation of hearing and the quality of what is heard.
聲 combines 殸 (a stone chime or musical instrument) with 耳 (ear), suggesting sound perceived by the ear. The top component 殸 itself is composed of 声 (sound) and 殳 (a hand holding a stick, implying striking), reinforcing the idea of producing sound.
The top part 殸 shows a hand striking a musical stone to produce sound, and the bottom 耳 is an ear receiving it. Together, 聲 is sound entering the ear.
For セイ, imagine a singer hitting a high note and the sound waves traveling straight to your ear: say -> セイ, and the voice fills the air.