I use duolingo on more than one device; the voice is different on each. There are some words pronounced quite diferently. I suspect that on my older android it's using a lower quality text-to-speech engine, because thats what it sounds like. I do not know this to be true, though.
I've been told that in standard everyday speech, they are indistinguishable because unstressed vowels get reduced (like "о" and "а" that become the same sound when unstressed).
When someone pronounces a word slowly for some reason however (maybe for example when asked "What did you say?"), then all vowels are pronounced differently, as in stressed position. Apparently native speakers make this "pronunciation shift" automatically, but still "think" unconsciously as if they were pronouncing definite "о" or "а". Except for some Russian dialects that always pronounce "о" as "о".
"о" & "а" should certainly be indistinguishable here: unless they are stressed or, possibly, at the end of the word, "о" & "а" are pronounced the same in standard Russian (and in the latter case, the difference is quite subtle, so in the first approximation you can ignore it as well). As for unstressed "е" vs "я" - I think I can hear the difference, but once again it's subtle. The context should take care of this.