It is possible for an audio stream on a camera to be played via the HDMI port through a SPD-152 decoder. This will only work when the SPD is in Standalone mode:
To do this, firstly enable audio for every profile the SPD-152 will use. This is done on the camera webpage:
On the SPD-152, enable audio for the camera channel:
A speaker icon will then appear for every channel where audio is turned on:
You can then turn on audio for this channel or mute it by clicking the speaker icon. Please note, only one audio channel can be played at a time.