Does 声 exist as a traditional character? I see that 声 has an entry in the Kangxi dictionary, but every site seems to define 声 as a simplification of 聲. Does 声 also exist as a traditional character? If not, then what's the glyph origin behind 殸?
Yes,「声」represented the word now written as「磬」(stone chimes).

「声」was originally part of「殸」(stone chimes), and this character later became「磬」. Later on, people began cutting off the components from「聲」(sound) to use「声」as a shorthand for「聲」(sound), but「磬」and「聲」represented different words.


  「聲」(sound) is comprised of「殸」(stone chimes being struck) and「耳」(ear).




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「声」depicts stone chimes「石」suspended with threads/string. In「殸」, a hand holding a striking implement「殳」was added to emphasise that the instrument is meant to be struck. Finally, another semantic「石」(rock, stone) was added on to「殸」, as a further emphasis that the instrument was made of stone.

In the modern form, the threads used for suspending the chimes is written as「士」.


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  「石」was originally a depiction of stone chimes, and its original meaning is stone, rock. A distinguishing symbol「口」was added later to mark the name of an ancient region called「石」, and is inherited in the modern form. The stone chimes component was later simplified into「厂」, then the strokes were further altered slightly into「丆」, resulting in the modern shape.




References:


季旭昇《說文新證》
黃德寬《古文字譜系疏證》
小學堂
中華語文知識庫・漢字源流
國學大師


郭沫若《甲骨文合集》
中國社會科學院考古研究所《殷周金文集成》