The volume of the click sounds (called Button1 and Button2) does not seem to be affected
by the "effects" volume slider in game (at least for me).

So in this archive there are 4 lower volume files for those 2 sounds,
as well as "mute" replacements and backups of the 2 original files.

1. To find the right one for you, you should have DW:Universe running.
2. Than Alt+TAB to the desktop and copy a version into the \Sounds\Effects folder, overwriting the original.
3. Click a button in game "Start new game" plays "button1.wav", closing the then open window again plays "button1.wav"
4. Repeat 2&3 until you find the right ones for you.

Have fun,
Locarnus (on the matrixgames forum)