Yes, you can soundproof existing walls by using a soundproofing material like mass loaded vinyl. However, it's important to note that adding a soundproofing material to your walls will only help to reduce the amount of noise that travels through them; it will not completely eliminate noise transmission. If you're looking for a more effective solution, you may need to consider renovating or replacing your existing walls with taller and thicker walls that are specifically designed for soundproofing.

There are a few ways to soundproof existing walls. One way is to add an extra layer of drywall using Green Glue, which can be found at most hardware stores. You can also use acoustic panels or insulation to help absorb the sound. If you're handy, you could also try adding some soundproofing foam or batting directly to the wall. Whichever method you choose, make sure that you seal all of the cracks and holes in order to get the best results. Good luck!