Start here http://www.tonymacx86.com/audio/143757-audio-realtek-alc-applehda-guide.html then go to here http://www.tonymacx86.com/audio/143752-no-audio-devices-realtek-alc-applehda-guide.html or http://www.tonymacx86.com/audio/143750-no-sound-realtek-alc-applehda-guide.html depending upon your specific problem/ if first post does not fix
Edit**
Is this on your laptop? You may have better support, in the Laptops section for your specific OS X version