"Các sở thú"
The final -c sounds silent in các in this recording. Is that standard?
No, it's not silent. Final consonants -c /k/, -ch /k/, -t /t/, -p /p/ in Vietnamese are unreleased stops. That means they're less-aspirated and pronounced quickly. So it may be difficult for you (a beginner) to hear and differentiate them at first.
more importantly don't pronounce them hard or it will sound incomprehensible or in this case an entirely different word
I don't hear any semblance of a /k/ in this one. My Southern Vietnamese girlfriend also seems to not pronounce the final consonants at all for certain words, like muốn. Not sure if it's regional, or just a slurred version of the "proper" pronunciation?
Because they are silent. You still shape the sound but you don't make the sound
I typed "các sở thủ" - had the tone of the last word wrong. In the past Duolingo showed me the mistake when I checked the answer (you have typos). That feature is gone since the "upgrade". Now I am told my answer is correct. No help with the tone problem. Thanks, Duolingo, for the "upgrade" that makes it makes your app less useful.