Translation:Please be careful
do you read the 오 as 아? Cause that's what I'm listening! Is it because its said too fast?
오 is 10000% of the time read as 오. One nice thing about Korean letters.
I hear it too, I think it's just a quirk with the text-to-speech system Duolingo uses. It should be pronounced as "오".
If you listen to it very carefully it is pronunced as "오" but I think the robotic voice makes it sound like "아"