In most buildings there isn’t a need for more noise insulation but they’re still noisy. You can instantly fix that by (drum roll) adding a foam backing to wall outlets and switches. Over the years these little pieces might have been removed or your apartment might appear perfect but for that noise caused by this piece of insulation thats easily discarded and not thought of anymore. Start with the noisiest room, as when your bedroom might back on to someone’s kitchen.
You can also stuff the opening with more insulation