Soundproofing isn’t usually required
Generally, no. California landlords are usually not required to soundproof an apartment or to guarantee that you will not hear normal noise from neighbors or the street. Ordinary sound transfer between units is typically considered a normal part of apartment living.
There can be exceptions. Some newer buildings must meet certain sound-insulation standards in their construction, and if a building failed to meet the code that applied when it was built, that could raise a different issue. Local rules can also vary. But an older building simply having thin walls or floors is usually not, by itself, a violation the landlord must fix.
The stronger tenant protections around noise usually come from the right to quiet enjoyment and from enforcing lease rules against unreasonable behavior — not from a duty to soundproof.
If you are dealing with noise, the more productive focus is often the source and whether it is unreasonable, rather than expecting the landlord to rebuild the walls. If serious, unreasonable noise is disrupting your home, a free, confidential review can help you understand which rights actually apply.
This is general information about California tenant rights, not legal advice. Every situation is different.