I plug my usb headset in and out frequently. I can select the headset as the main channel and control it using volume keys on my headset when plugged, but if I unplug it, volume control keys stop working and the tray icon shows white square with an "?". Pulseaudio's standard volume mixer has a fallback device option. I wonder how much effort would it be to implement such an option in kmix, e.g. when I plug my usb headset out, the main channel and the volume controls fall back to either the device I have specified (fallback checkbox on each sound device) or to some kind of default device. The idea is that volume contols don't stop working if a headset is unplugged. Reproducible: Always
There is already a fallback implemented. KMix even shows a small popup to indicate the fallback. Strangely enough it does not seem to work for you. I haven't myself tried it for some time, though - I'll try to test it once I get hold of a USB device.
*** Bug 303020 has been marked as a duplicate of this bug. ***
Yes, it appears to have been fixed. Thank you!