SUMMARY I've updated kmix to version 4.6 and since then scrolling with the mouse wheel over a slider doesn't work anymore. I'm aware of https://bugs.kde.org/show_bug.cgi?id=409325 and see it as advantage, not as a bug and I fear someone removed the functionality completely, while trying to fix this bug.
The referenced bug refers to the Plasma volume control applet and not KMix. Mouse wheel scrolling over a slider currently works on the volume popup sliders, on the system tray icon (without the popup open), and over the main window sliders. Please reopen with any more information that you can provide if still a problem.