Hi guys I have already had this problem in the past with the same MBP. I successfully reset the PRAM, but this time its not working. Its a 2012 version MBP originally ran Lion, then Mavericks now running Yosemite OSX. Movies with my headphones have always worked but then one day the volume stopped working on my MBP. Only worked when I plugged in my headphones, so it didnt come out of the speakers. I tried to reset the PRAM a few times recently, but the MBP struggled to do it (sounded and felt awful) and the start up screen/grey screen just kept flashing simultaneously. Any other ideas?