Max2Play and HDMI ARC on Ikea Uppleva sound system 2.1

Hello guys,

I am a enthusiastic user of this software on raspberry pi.

I have 5 of them connected by bluetooth, usb speakers of hifiberry Digi+.

I came to buy an uppleva sound system by ikea.
http://www.ikea.com/cn/en/catalog/categories/departments/living_room/23139/

and if you want to have an overview of the connections :
http://res.ikeaddict.com/products/u/uppleva-systeme-audio-__1083443-2_pub/uppleva-systeme-audio-__1083443-2_pub-0.jpg

You can connect 4 wait in this system :
– jack
– rca
– optical input
– bluetooth
– hdmi

Now for some reasons I would like to use the HDMI to receive music from my Max2play device.
– jack/rca = sound too weak, even with an external usb audio card
– optical input : ok but needs a hifiberry digi+ card for each system, and I plan to put 5 at home…
– bluetooth : would like to keep the ability to connect directly a phone to the system by bluetooth, WAF requirement…

Now follow me.
If I connect the sound system to the TV using the ARC HDMI port on the TV => works perfectly, the sound streams from the tv to the sound system

if I connect the raspberry to the TV by HDMI (itself still connected to sound system) => works perfectly, the sound streams from raspberry, to the tv, to the sound system

if I connect the rasberry directly to the sound system (through HDMI), it does not work, no sound coming out.
I tried to boot in this configuration => not working
I tried to switch from configuration 2 to the target = not working.

I use a plain vanilla image from the site.

Do one of your guys see any reasons why the signal coming from the raspberry through HDMI differs from the signal coming from an ARC HDMI port behind a TV ?

Thanks in advance

Guigeek