Everything is still default. It started after I updated the Mede8er (in the language selection, time, ...).
I don't have any special sound cables or stuff like that. Everything runs trough one HDMI cable.
With the sound I have a feeling that it happens after I transfered files over my network to the HDD after which sounds gets disabled and I need to reboot the device.
About the black flashes: I don't know, I also had it with the V2 software.