if you do want to add a layer of additional sound insulation, i do recommend a product (i have used) which will give you maximum sound attenuation for the depth it will use, that is Quiet Rock 545. it’s an inch and 1/4 thick but will perform like 5 sheets of 5/8" sheetrock. it is heavy and hard to cut for your contractor, but it will do the job.
a layer of green glue between the Quiet Rock and current wall will also help insulating.
these 2 products should get the job done.....and you should be Led Zeppelin or Beethoven’s 9th ready to boogie.
if there is a crawl space underneath the townhouses you might also erect a barrier there, but that would need to be something engineered for that space. not sure what is appropriate there.