Mini Display Port to HDMI - no sound
It's possible your Mac doesn't support audio over Mini Display Port:

These Apple computers supply multichannel audio (up to 8 channels) and video signals over Mini DisplayPort:

MacBook Pro (Retina, 13-inch, Late 2012 and later) 1
MacBook Pro (Retina, 15-inch, Early 2013)
MacBook Pro (Retina, Mid 2012)1
Mac mini (Mid 2010 and later)1
Mac Pro (Mid 2010) 
MacBook (Mid 2010 and later)
MacBook Pro (Mid 2010 and later)
MacBook Air (Late 2010 and later)
iMac (Late 2009 and later)

Also, have a read of this question: 
Does this Mini DisplayPort to HDMI connector output audio?