Q:

Voltage of smartphone audio jack

Strangly when i insert an audio to audio cabel into the the smarthphone audio jack the multimeter shows me max. 0.1 V.
But if the cabel is just half inserted in the audio jack the multimeter shows me a Voltage of 2.4V
Can someone explain how this is possible and how much Voltage i can exepct from and smarthphone audio jack?
I am mearsuring between ground and mic on a headset connector.

A:

Depending on the standard used, the microphone pin will have a voltage (generally with a large resistance in series). This is needed to allow the microphone circuit to work. The large resistance in series is there so that when the microphone pin is shorted out by the headphone cable, it doesn't damage anything (almost no current can flow and the headphone is not damaged). Since the current draw of the microphone electronics is very low, when connected it does not cause a large voltage drop allowing the microphone to work.
Here is a similar question that might shed some light on this matter: 
Microphone voltage on a 3,5mm PC microphone/headphone jack

