Rim-Brake pads creaking in still state
What you are hearing in my opinion is the squeak of the rubber pads gripping the rim.  Notice that the pads do not close into and on the rim from a perfectly lateral, 90° approach.  The path of the pad takes a slight arc into the contact point with the rim.  At the contact point as one continues to squeeze the lever and modulate the braking force, the pads continue that slightly arc'd path which leads to some slight sliding on the rim.  The displacement of the rubber and the arc'd path of the pad combine to cause a brief squeak as the rubber settles into the rim.  This is a normal observation and actually an indicator that the rubber of the pads is in good shape and not weathered or hardened.  Some might infer there may be some contamination that's causing the squeak.  Possibly, and this can be corrected by cleaning the braking surface of the rim with alcohol and roughing up the pads' braking surface with some sand paper or bastard file.  At any rate, this is a fairly common sound and a normal finding that does not affect braking performance in any way.