you can activate it manually , edit guisettings.xml.
Code
<setting id="audiooutput.passthrough">true</setting>
<setting id="audiooutput.passthroughdevice">ALSA:hdmi:CARD=HDMI,DEV=0</setting>
<setting id="audiooutput.ac3passthrough" default="true">true</setting>
<setting id="audiooutput.ac3transcode" default="true">false</setting>
<setting id="audiooutput.eac3passthrough">true</setting>
<setting id="audiooutput.dtspassthrough">true</setting>
<setting id="audiooutput.truehdpassthrough">true</setting>
<setting id="audiooutput.dtshdpassthrough">true</setting>
<setting id="audiooutput.dtshdcorefallback" default="true">true</setting>