The only way I got even close is through the work of everyone else here, and I'm really just piecing together things everyone else has said so consider this a "group solve", but I believe the answer is:
Dr. Worm (a song by They Might be Giants). Everything is a reference to lyrics from that song.
The QR codes say:
A REAL ONE I AM NOT
YET REAL I AM
Well:
The lyrics for Dr. worm say:
I'm not a real doctor,
But I am a real worm;
The QR codes also say:
Just come turn the knob
and things get interesting
and
Dr. Worm lyrics:
I'm interested in things.
How the clues fit in:
clue 1:
Last door at the end of the hall
explanation:
The last letter of the clues for each QR code are TMBG - initials for They Might Be Giants.
clue 2:
As you listen at the door, you can hear someone playing an instrument
explanation:
I like to play the drums.
clue 3:
The neighbor's door has a post-it note stuck to it with the question "Why does the sun shine?"
explanation:
Another song by TMBG
original hint 1:
There's also a doorbell, but no one answered when you pushed it.
explanation:
I'll leave the front un-locked 'cause I can't Hear the doorbell
original hint 2:
They really seem to enjoy banging those drums!
explanation:
I like to play the drums.
original hint 3:
Istanbul/Constantinople
explanation:
Another song by TMBG