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).
「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」.
「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.
声 is a pictograph of an old stone instrument. The radical 殳 is a hand hitting something with a stick. The ear 耳 below 聲, must represent the hearing of the sound from this instrument being played. See the book: "China: Empire of Living Symbols" by Cecilia Lindqvist. If I remember correctly it is in there.
殸 led to 聲, but was not composed from 声; it's not like evolution to ever more complexity, sometimes a component is isolated from more complex forms. 声 has been used for a thousand years, if that is "traditional" enough.