How to hide audio volume pop over?
I found an answer here

Basically it is going to 


  /System/Library/LoginPlugins/BezelServices.loginPlugin/Contents/Resources/BezelUI/HiDPI/. 


You'll see there a lot of pdf-pictures, one of them volume.pdf. You'll want to overwrite that one (backup the old one!) with an empty .pdf. You can also overwrite the mute.pdf, but I dont think that is necessary in movies.

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)