No sound in external view after startup

HP Reverb G2
I recently started having the following problem in VR:
When I am in the outside view, the sound is present.
But as soon as the airplane takes off from the runway, the sound in the outside view has disappeared.
The sound in the interior view remains and is normal.

Does anyone have an idea what the problem could be?

Addendum:
When I touch down on the runway again after take-off, the sound is back.