What's the purpose of the inductor in this microphone biasing schematic?
The value of 15 nanohenry is about the inductance of 15 millimeters of wire.

The 15 pF (suspiciously the same numbers as the inductor) is more than casual stray parasitic, but in the range.

The resonance is in the hundreds of megahertz; with no dampening shown, the usefulness of 15nH and 15pF is doubtful.

I'd expect these to be residual values, and if PCB solder pads exist, the L and C are provided as optional EMI filtering.

The microphone cable may have Positive and Negative pins. Are you using those?