About the "?" button

Windows 7 64-bit - Audacity 2.2.0-beta-Sep 1 2017 - .exe-installer
The “?” button links to help/manual/man/interface_preferences.html on lokal system.

I dont think a “?” is good for this. It does not really tell the user when to expect or rather give the user a question.
“Help” would properly be better.

Also the button is not the same height as the other buttons because of the use of a image.

It should do really, as it is very widely used in software. It really is a very commonly used symbol. For example, I am typing this into Firefox, which uses a “?” for the help menu. Try searching Google Images for “Help Icon”.

The reason for a “?” rather than the word “Help” was so that there would be one common type of Help button that could be used, even when there is not much space available.

I agree that the button doesn’t look very pretty in your Windows 7 screenshot, but I think it looks better in Windows 10, Mac OS Sierra and current versions of Ubuntu.