Thanks for the support.
Happy to have solved, leave what i found for the next.
1) hdmi audio on imx6 boundary-devices Linux 3.10.17 works out of the box, just selecting it in sound soc menuconfig.
* BUT *
using devicetree, video=mxcfbX kernel params must match with consistent mxcfb entries into the devicetree.
Otherwise, a wrong hdmi resolution can be auto-probed, and audio is inhibited.
So, hdmi audio wors only if a suitable resolution is set
In my case it is:
mxc_sdc_fb fb.30: 1920x1080 h_sync,r,l: 44,88,148 v_sync,l,u: 5,4,36 pixclock=148500000 Hz
Best regards
angelo