noun
A soft, low voice, often used when speaking secretly or quietly.
彼女はささやき声で秘密を教えてくれた。
She told me the secret in a whisper.
The verb 'to whisper'; ささやき声 is the noun form referring to the whispering voice itself.
Compound of ささやき (whisper, from verb ささやく) + 声 (voice).