Please, provide a way to synchronize audio volume adjustment for multiple (all) devices. Ideally, when I press Volume Up/Down hotkeys, I'd like the volume of all my output devices to be adjusted by the same amount. Currently, the applet only adjusts the device that is selected as "Default", and switching it to another device is cumbersome. This can be useful if one has multiple audio systems (e.g. a 5.1 set and headphones) connected to different devices. I'm most interested in output volume, but it might be convenient for input as well. I'm not sure what version, but I think it used to behave that way some time ago.
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This is still a valid concern with Plasma 6, I think it's worth considering something like a checkbox / radio slider for adjusting all volumes at once (or maybe a checkbox per output)