sudo nano /boot/config.txt
hdmi_force_hotplug=1 hdmi_drive=2
References :
https://www.element14.com/community/thread/18205/l/how-to-get-sound-over-hdmi-working-when-using-a-tv?displayFullThread=true
http://raspberrypi.stackexchange.com/questions/2169/how-do-i-force-the-raspberry-pi-to-turn-on-hdmi