Can not adjust the sound with the keyboard

Hello,

So here for 2-3 days, the sound of my Macbook starts beuguer (buy in October 2014). So when starting the mac when I listen to music all goes well, I can adjust the sound using the keys on my keyboard. But as soon as I plug in headphones, not set I have to go through the system preferences. By unplugging the problem remains the same on his fate, but impossible to resolve through the keyboard. I tried to put the volume icon on the top bar but even by setting it does not work here. The problem resolves itself after 30 minutes-1 hour. It's very annoying [IMG]. Thank you in advance for your answers.