Volume reduces automatically after pluging headset into macbook pro I got a MacBook Pro 13'' early 2013 with OS X 10.10.5 installed and SONY MDR-1A headphones with a headset cable.

When using "headphone" cable everything works as expected. When I use "headset" plugin (the one with 4 grooves on a jack and a mic on a cord) the volume slightly reduces after 1 or 2 seconds.

Can someone explain this behaviour and tell me if it can be fixed?!
Headphones volume level is separate from the built-in speaker output. For example you can set the speakers' volume to 0 before plugging in headphones and then adjusting the headphones' volume to something different. When the headphones cable is unplugged, the system goes back to the previous volume value.

If I remember correctly, recent Macs also have a impedance sensor in the headphone jack, so that you can have different volume levels for different headphones based on impedance.