Call for perspectives on a passage

I have been working on a woodwind trio for which I have written a passage which has been confusing at best. As I tend to be guided by my ear when writing, I sometimes write things that sound aligned with my expectations but which I do not entirely understand! This seems to be amplified when I use chromaticism, as in this example. 

I think I now understand, but I am a bit stuck on what to write in the inner voice. 

My understanding is that the first two bars mark the dominant of A minor, with a D sharp chromatic inflection in the bass in the second but which resolves in the third beat. And the third bar is I in A minor, with a C sharp chromatic inflection implying d minor but resolving to an A minor chord. This proceeds to the V7 of A minor in preparation for the cadence in Am (but in fact resolves to F major). 

The upper voice descends chromatically in such a way that complicates the harmony and opportunities to fill in the middle voice. My low experience with chromaticism really shows its true colors here. 

I have attached the audio - the passage in question is from the 50 second mark. 

Would be great to hear other's perspectives.