Audio device preferences in Plasma are set in: System Settings -> Multimedia -> Audio and Video In this dialog we can select what device is preferred for music. However, Elisa outputs sound into default device.
The configuration panel you are talking about is for the Phonon API only. Elisa is using another API. We use the Qt multimedia module to have a multi platform single API to target Linux desktops, Windows desktops and Android. On Linux desktops, it is going through gstreamer. Depending on your distribution and local configuration, it may go directly to hardware or go through pulseaudio. Those modules have their own way to configure the audio output. We do plan to explore using Vlc API directly on desktops. This will also not fix your problem. I am sorry but we cannot do much with respect to the settings of this configuration panel.