Check this: https://discourse.osmc.tv/t/no-headphone-audio-how-to-configure-in-osmc/3790
http://openelec.tv/forum/124-raspberry-pi/75330-rpi-2-analog-audio-jack-static-noise
http://kodi.wiki/view/Raspberry_Pi_FAQ#Video_and_audio_output_on_the_Raspberry_Pi