He told me the secret in a low voice.
Please speak in a quiet voice in the library.
She answered in a low voice.
Please talk quietly in the library.
When the topic turned to a secret, his voice dropped to a whisper.
loud voice; shout
小声 describes speaking in a low, quiet voice, the opposite of 声高, which describes speaking loudly and assertively.
Compound of 小 (こ, small) and 声 (こえ, voice). The reading こごえ is a standard rendaku (sequential voicing) of こえ.