Thank you, @Litjan!
Even though I got that solved, I still suffer a problem many others have already had, according to google research:
My sounds are partly broken: Some functions sound weird, for example stall warning test or take-off config warning or give me no sound at all, for example the gear handle. My XP sounds are reduced by the IXEG 737. I already tried changing the IXEG sounds when bumping to the left of the screen, I deactivated all plugins apart from X-Aviation and Gizmo, re-installed, checked, whether my system and X Plane sounds match and so on...
...what else can I do?
Best regards!
Patrick