Thank you for your feed back I agree, and I followed the I2S sync error to some other website forums… They did indeed recomend kernel upgrade but it was for a hifi berry and it dated back 1 year on kernel 3.7 … so I thought (maybe wrongly) that the kernel would be fixed by now. And also that the kernel was “fine tuned” in zynthian for low latency and so I was “wrongly” afraid that updating the kernel would not be too good.
I am doing the update while writing this down and will update with my experience. I am sorry to ask all those questions… Still I hope the trace in the forum will help more people in the future.
UPDATE: upgrade as you recommended seems to have proceeded successfully, but unfortunately the I2S SYNC error is still there and no sound :
root@zynthian:~# aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: audioinjectorpi [audioinjector-pi-soundcard], device 0: AudioInjector audio wm8731-hifi-0 
Subdevice #0: subdevice #0
Playback device is default
Stream parameters are 48000Hz, S16_LE, 1 channels
Using 16 octaves of pink noise
Playback open error: -16,Device or resource busy
and it is still the same even if I systemctl stop zynthian.
I read in an other thread that you also have an audio-injector, but did you manage to make it work with zynthian yet?