Detect if bud earphones are in ear
If the ear buds were perfectly sealed and inserted with a perfect seal, there may be slight changes in impedance measurable by swept frequency impedance analyzer, but since the air load pressure is slight compared to the source pressure, it might be very small like putting a 100k resistor in parallel with 100 ohms. 

If you sealed the ear bud up tight, it would be more noticeable without much volume of air.  If the ear bud was submerged in water for example ( not recommended) then a less compliant load would increase the current wave somewhat and the pressure and make it more measurable from the change.

The compliance or change in diaphragm volume relative to the volume of air in the ear canal is what might affect the impedance.  Typically The impedance goes down to 50% at DC or if the diaphragm was prevented from displacement with something.  The stall current in DC linear voice coil can be as low as 10% of the AC impedance (at some rated power) but for speakers it is closer 50%.