etc/mixer_paths.xml

<mixer>
    <!-- Enable HDMI as default audio out for db845c -->
    <path name="default_db845c">
        <ctl name="QUAT_MI2S_RX Audio Mixer MultiMedia1" value="1" />
    </path>

    <!-- Enable HDMI as default audio out for rb5 -->
    <path name="default_rb5">
        <ctl name="TERT_MI2S_RX Audio Mixer MultiMedia1" value="1" />
    </path>
</mixer>