how can I Soundproof a room without wall modifications
Update: with all the concerns you can also use what is called Mass Loaded Vinyl which is normally used in a non abrasive method which is good for places such as rentals, flats, homes etc.


  Mass Loaded Vinyl is used to reduce airborne sound transmission
  through walls, floors and ceilings. They are typically comprised of  a
  limp-mass material sound barrier made of high-temperature fused vinyl
  and no lead fillers.  With a typical weight of 1 lb. per square foot,
  these barriers are as heavy as lead, but only 1/8" thick. This
  improves the sound transmission loss (STC) of a construction assembly
  without losing valuable space. It may be used in new and retrofit
  soundproofing construction. The standard offering is a non-reinforced
  barrier for use in typical construction. Other configurations include
  lag and wrap materials for pipes and ducting, a transparent version to
  allow line of site or light, a surface mount option and a reinforced
  version that may be suspended. These materials are often combined with
  other materials in soundproofing applications to help provide a
  complete sound blocking solution.


You can check it out here. I am in no way affiliated with acoustical solutions.

https://acousticalsolutions.com/product-category/sound-barriers/mass-loaded-vinyl/



You can use soundproofing foam, this is how we soundproof recording studios or if you want something cheaper, you can use egg crate flats. Each will require at least some glue. 

foam



egg crate flats