Hi,
Originally this fault was observed on Openelec 6.95.3 but it is present in Librelec 7.0.1 as well. I have two AMD based HTPC at home and both suffer by this fault.
After the system wakes up from sleep via CEC there is no sound on the HDMI output. Recovery is to go to Settings/System menu. Nothing needs to be changed. When I enter to System submenu the audio comes back immediatelly.