This was to remove the glue that stayed on the walls. Just spray ahead... Let sit, spray again, let sit... Start scraping
Use a wallpaper scorer to make small holes in the old wallpaper. Next, soak the wallpaper with water. The holes will make sure the wallpaper is fully soaked in and out. Start stripping off the wallpaper.
This may sound easy but if this is the first time, you might end up with a bad result.
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Spray foam will do a lot to help retain heat/cooling the interior, but as a soundproofing agent, it is only so-so.
Contrary to other answers, the properties of spray foam are not designed to stop sound https://www.quiethome.life/spray-foam-vs-fiberglass-insulation/ . They will mitigate sound transfer to a point, but mostly higher frequencies.
Low frequencies will still penetrate into adjoining space.
To truly soundproof a room, it requires structural alterations beyond the affordability of most people, going into the thousands of dollars, even over ten grand to twenty grand, depending upon the size of the room.
Alternatively, stick with the traditional method of warm water and a wide bristly brush. Soak the wall paper well and allow to be absorbed for a minute or two and it should come off no problem with a scraper. Apply more water if the paper is proving to be stubborn. Apply the water generously but stop short of flooding the room!