Electronic – Would an audio signal be degraded by adding a 0.45 ohm resistor

In regular line level audio, DC voltage does not serve a purpose.

It's conceivable that some devices may use a DC offset in order to detect activity and to go into standby - or turn on - depending on whether there's a DC offset or not, without having to depend on an actual audio signal to be present at all times. But I'm not aware of any standard for this.

Audio output circuitry usually does contain DC blocking capacitors.

(But not always; certain high end "audiophile" power amplifiers do not. With these amplifiers, any DC voltage offset on the input signal can easily blow the speakers. Yes, it's a dumb design.)

A headphone jack does not output line level audio though; it's a low(er) impedance output designed to drive tiny speakers. If the DC voltage on the output of your phone does serve any purpose, I'm guessing that it might have to do with the small remote control for receiving and ending calls, and changing the volume, often found on the cord, although there's often a separate wire for that (mini-jack with four conductors).

Your schematic reduces to

^{simulate this circuit – Schematic created using CircuitLab}

with mic left, iPhone right, and the sleeves being ground, and Ring2 being the mic connection according to the CTIA standard. The rest of the connections (Ring on the mic, Ring1 and Tip on the iPhone side) are left unconnected.

Your microphone is an electret one, which probably means it's hard for the iPhone to detect the presence of the microphone simply by measuring the resistance across it.

If you imagine the mic away, then you just "shorted" the microphone input of the iPhone to its ground connection with about 1.7 kΩ resistance, which might just be in the right ballpark to tell the iPhone there's an external active microphone attached.

Of course, you don't just need the microphone to be there, you also need it to deliver sound. My guess is that the sensitivity (volts per sound pressure) of the microphone is relatively high, and that the sink impedance of the microphone input on the iPhone is relatively low: In that case, the array of resistors forms a current divider, which in turn brings the voltages at the iPhone to the level it can work with.