Getting opposite expected voltage with electret microphone
If you look here you will see that the electret microphone secretly contains a FET ! It must because without it it is almost impossible to use such a microphone as it is a very small capacitor.

The circuit that you have should be OK but you can try lowering that 15 kohm resistor to 5.6 kohms. Don't worry too much about the DC level.

Do use a capacitor (1 uF) to connect to your audio input (unless that already has a coupling capacitor).

Due to the FET, measuring the DC resistance of the electret microphone is meaningless. In my experience they are also quite hard to break so my guess is that yours will still work.