The volume sliders in the panel are too hard to click on, because the clickable area is too small. It is well known (as Fitts' (1954) Law) that it's much easier to use a mouse if the target area is big. Specifically, the sliders in kmix are vertical, and are very narrow horizontally. Please make them wider (horizontally) so that they are easy to click on. Reproducible: Always
This is a good article with good advice: http://www.smashingmagazine.com/2012/12/04/fittss-law-and-user-experience/
The sliders are the same size and appearance as all other widget controls, their size is controlled by the current style. Not intending to change that, but patches are welcome.