For example, I had many VLC-player instances and browser open. I then opened The Battle for Wesnoth and wanted to adjust its volume through the mixer.

However, I couldn't see it and at first I didn't know why. Then I realized that I had to resize 

Reproducible: Always

Steps to Reproduce:
1. Open many applications that use sound (like many VLC-player instances, a web browser, a game etc.)
2. Open Audio Volume Settings

Actual Results:  
If the Audio Volume Settings -window is small, you don't see all the applications and there is no scrollbar.

You can resize the Audio Volume Settings, but in my case I had to resize the window so much that it covered 70 % of my vertical space. At first I didn't even know that applications were missing because of lack of space.

Expected Results:  
It would be nice to have a scrollbar when there are many (sound) applications open.

Kubuntu 16.04
Plasma 5.5
Qt 5.5.1