Click here, it will expand the bottom section where the click level is located.
Also, for those above asking for more click routing options etc, it is possible to render the Audio/Midi click to a track which then gives you many more routing options, the function is under the ‘Project’ then ‘Signature Track’ then ‘Render between locations’. 
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Wow, this is very nice to have! Although I must say it wasn’t super easy to get to!
Now using Cubase 13 (Nuendo 13). Intuitively, I was looking at my transport bar at the bottom screen my screen, where I use the “Click & Count-In & Click Pattern” feature to easily see and access metronome settings (see first screenshot below), and was looking for a little “triangle” fader knob, just like in the Audio Editor window, for the Audition Volume (see second screenshot below). There’s in fact a little triangle handle just aside the Audition Volume control that pops up a little fader when you click on it, to adjust the volume at which playback is auditioned within the Audio Editor.
Screenshot #1:
Screenshot #2:
Would indeed, I believe, make a very nice little addition, as metronome “blasts” happen all the time, just for example when inadvertently hitting the “C” letter on the keyboard and going “waahhhhhh” when auditioning low level material loudly (just to make sure, don’t want to remove the shortcut “C”, use it all the time).