Maybe this is not the intentional nor desirable workflow, but for a moment I thought that it would be possible to drag and drop one or multiple AudioSources on the Audio Group in Tree Mixer or Mixer channel view - please see attached screen shot.

My reference is Ableton Live where you can change a sound or instrument by dragging it directly on the mixer channel.

On the other hand, user can select multiple sound objects and then change the Audio Group in the Inspector, which works nicely.