Flash mutes HDMI audio out

4.4.0.18 - Xenial 16.04 x64 on Asus Zenbook

Video and Audio output via HDMI works fine until I open flash video playback.
In this case, the computer seems to tu into a muted mode where not even the "sound test" can be heard anymore, as well as any other sound that worked fine before.
PulseAudio Volume Control still displays the audio output (volume going up and down). Selected mode is HDMI Stereo.
A restart of the TV does not work.

As soon as the window with flash player is closed again, audio retus to normal.

aplay -l outputs:

card 0: PCH [HDA Intel PCH], device 0: ALC269VB Analog [ALC269VB Analog]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: PCH [HDA Intel PCH], device 3: HDMI 0 [HDMI 0]
  Subdevices: 1/1
  Subdevice #0: subdevice #0