Why are they both whispering?
"I want you to try on a school uniform. I think you'll look good in it." Koichi whispered to Viet in my fantasy.
The man started speaking in a low voice.
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.