It's the wall of speakers in the live sound room as opposed to the studio monitors on display (usually in the keyboard area).
The wall is also supposed to be "live" i.e. ready to demo using their switching/routing system, and you are supposed to be able to hook up a playback device and listen through whatever combination of speakers and subwoofers you like.
I just celebrated my 49th anniversary working in the industry, and have called on GC stores in a variety of capacities over the years. I got my start at a store in Texas that did more business out of one location than the 5 GC's that were in existence back then, FWIW.
I actually aborted my last GC mystery shop (at a location I have visited on behalf of a manufacturer). I was able to get my phone to play tunes via Bluetooth over the sound wall, and made a total racket switching speakers for a good couple of minutes with no offer of assistance.
Having experienced something similar when killing time (making all kinds of noise in the keyboard area while the sales loser kept their face buried in their computer screen), I decided I was biased and opted not to go through the hassle of trying to engage.
How that location stays going is beyond me... it's quite possibly the worst one I've been in out of dozens and dozens of visits.
Have synthesizers, will travel...