It should be accepted without articles like "a" or "the". Who knows if these are just labels or from a list? There's no context here to distinguish.
So, this "kk" combination is pronounced as "chk"?
No, it's pronounced as kk.
The audio is wrong. Listen to the pronunciation of the word miękki.
Well, I hear it clearly right.
If you listen closely to the audio provided by Duo, you'll hear it pronounced as in "mięCHki".
Maybe you have some problems with audio? Especially on a phone? I, a native, hear 100% right.
I think I hear just a little hint of some 'ch' sound. Hearing is very subjective, especially when you actually try to hear something precisely.
Why are articles necessary here?
Oversight, added the possibility to omit them.