How can I disable/hide this automatic volume level pop over that appears when changing the sound level. Is there switch or secret flag to not show this when I change the volume using the volume keys on the keyboard (Macbook)?
I found an answer here
Basically it is going to
You'll see there a lot of pdf-pictures, one of them
When you open this PDF in Preview, you will see a white square, by default, Preview will show transparency as 'white'. As the illustration is white, and the transparency is white, you will see a white square instead of the white volume logo. You can turn the white transparency off. Go to View, Show Image Background (AltCommandB)