In the middle of slow etudes, Berg provided added volume (6)
I believe the answer is:
louder
If this is the answer I cannot explain how the clue works.
Can you help me to learn more?
(Other definitions for louder that I've seen before include "Sound of greater volume" , "'Actions speak . . . . . . than words'" , "Noisier or gaudier" , "More noisy or bright" .)