Somewhere dormant in our systems is ancient support for Pueblo and its sounds per location feature, so we make could something like this work for the webclient. That's a good 60-75% of players at any given moment.
Maybe different kinds of music to communicate general locality? I could see 6-8 key tracks as a starting point with 2 states for each city sector - outdoor (something faster tempo, moving quickly thru streets) and indoor (something slower pace, suitable for 1 on 1 RP).
Red sector - start with a unifying sound that brings chaos and violence to mind - something fast pace for outside and something slower for inside
Gold sector - still somewhat chaotic, but no longer violent, more positive
Green sector - much more harmonious, nothing negative
Blue - classical, heavenly (if you do blue - gets a tiny amount of visitors really)
After that, we could implement it as an MVP feature and get more specific as inspiration lets happen. As time goes, if you (or someone else) gets an idea for a unique sound to give a corporation or territory in the red sector, we could apply it to those places in turn.