γκρι the colour doesn't change in plural but you can use the adjective γκρίζος (masc.)-γκρίζα (fem.) -γκρίζο (neut.) =grey/γκρι so in plural you can say οι γκρίζοι ελέφαντες.
The woman says ο (singular article) but ελέφαντες (plural). Listened to it thrice after being marked wrong for ο γκρι ελέφαντες (fair enough) but it IS what is being said.
I've listened to the sentence's slow and regular audio, but I couldn't spot the problem you're talking about. It sounds like ελέφαντας to me, in both cases. I've asked other mods to listen to it as well just to make sure, currently waiting for their feedback, although as a native speaker, I think I would've spotted that ε. ._.
After playing and replaying the audio numerous times, I think it's not as clear an α as it could be; there is a very slight æ somewhere in there. I can only just trace it with headphones, not speakers.
However, as a native Greek speaker myself, I don't know that I would have noticed it had ErikdenB not brought it up. The grammatical clue in the article "Ο" is so strong that everything else falls easily into place. :) It is easy for us to ignore/miss any unusual vowel sounds that do not conform to what we expect to hear because Greek only has [a], [e], [i], [o], [u].
Why "g" sound is not the same as in "γυναίκα"? Are there two different "g" sounds in Greek?
When γ is followed by a vowel, it's always pronounced like y in yes (γυναίκα, γάτα, γέλιο).
When γ is followen by κ or another γ, they produce a sound similar to g in garden (αγκαλιά, αγγούρι, γκρι).