Thanks for your quick answer.
The HDMI cable I use is a 1.4a, sorry for the wrong information in my first post.
However, I installed the new driver what updated my Intel Audio Driver from 3135 to 3137. Unfortunately the Intel Audio device still disappears in both, the device manager and the list of playback devices, when switching between my monitor and the LG TV. A system restart brings everything back to normal and the audio output works fine but I like using the display control (Ctrl+Alt+F1 by default) which results in the issue mentioned above.
I'll search for an bios update soon and report the result, because the disappearance of the audio device seems to be a bios issue the more I think about it.