If the voltage comes from the audio jack, it is the DC used as the bias voltage (for electret microphones). Typically 1.5 volts, which is likely way higher than

If the voltage comes from the audio jack, it is the DC used as the bias voltage (for electret microphones). Typically 1.5 volts, which is likely way higher than the AC signal range of the input (likely to in the order of <100 mVp-p).