MacPro volume is quiet

Hi!

I have a macbook pro, had it for just a few months less than 2 years now, and for the past couple months the sound has been really low, the volume is turned up 100% but it sounds like its less than half a lot of the time, but itunes volume works fine. When I have earphones in it is normal but watching out loud isnt. Its on everything i do, youtube videos, movies, anything with sound.. my girlfriend just bought a mac 6 months ago, so we tested it out by watching the same videos and her volume was really loud, the way it should be.