noun, の adjective
A small voice is so small that you can hardly hear it. It's a low voice or a whisper.
The readings for this word are both kun'yomi readings. You've learned both of these readings by now so you should be able to read this on your own. Just watch out because, like in 大声, the こえ becomes ごえ thanks to our old friend rendaku.
to say in a low voice
to talk in a low voice
to mutter in a low voice
"Thank you," I whispered.
The boy read his composition with a low voice.
The teachers were talking very quietly.