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?!