First you can check if you can get some sound working 
List the devices and check the id of the card and device for hdmi, for example :
aplay -l
**** List of PLAYBACK Hardware Devices ****
card [B]0[/B]: HDMI [HDA Intel HDMI], device[B] 3[/B]: HDMI 0 [HDMI 0]
Subdevices: 1/1
Subdevice #0: subdevice #0
card [B]0[/B]: HDMI [HDA Intel HDMI], device [B]7[/B]: HDMI 1 [HDMI 1]
Subdevices: 1/1
Subdevice #0: subdevice #0
card[B] 0[/B]: HDMI [HDA Intel HDMI], device [B]8[/B]: HDMI 2 [HDMI 2]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 1: PCH [HDA Intel PCH], device 0: ALC283 Analog [ALC283 Analog]
Subdevices: 1/1
Subdevice #0: subdevice #0
Find some wav to play :
find /storage/roms -name *.wav
or get one online
wget "http://samples.mplayerhq.hu/A-codecs/wavpcm/ahh.wav" -O sound.wav
Then try to play some sound with hdmi devices : aplay -D plughw:card_id,device_id
aplay -D plughw:0,3 sound.wav
aplay -D plughw:0,7 sound.wav
aplay -D plughw:0,8 sound.wav