声 and 音: voices and sources of sound
Distinguish human or animal vocal sounds from the broader sound category.
Voices and animal calls
声 (こえ) means a voice or an animal's call. 「鳥の声が聞こえます」 says birds can be heard calling.
「小さな声で話しました」 means speaking in a quiet voice. It does not specify a small amount of speech.
Sound in general
音 (おと) means sound. 「雨の音を聞いていました」 describes listening to the rain.
「隣の部屋から大きな音がしました」 says a loud sound came from the next room. It does not identify who caused it.
What is evaluated and what caused it
「声は聞こえますが、言葉は分かりません」 says the voice is audible but the words are not understood. Separate hearing from understanding.
「声ではなく、楽器の音でした」 corrects the source to an instrument rather than a voice.
Does 「小さな声で話しました」 mean saying only a little?
Show the answer and why
No. It describes low vocal volume; the amount said is separate.