Is my landlord allowed to just walk into my apartment unannounced?

No, you landlord most certainly CANNOT do this. As a renter, you pay for possession of the apartment; that possession includes the exclusion of the landlord's right to enter the premises at will.

There are some times when a  landlord may enter,  of course. A landlord may enter at reasonable times, on reasonable notice (generally held to be 24 hours) for routine maintenance or inspection, or to show the premises to re-rent it or sell the building. A landlord may enter without notice for a true emergency: for example, a fire, a significant water leak, a gas leak, major electrial problems, or a tenant's health emergency (e.g. to let the paramedics in)--but those were not the situation you describe. A landlord may not enter at will to ask people to be quiet; he can certainly knock on the door and speak to them if they answer or let him in, or else call, email, text, etc. asking them to be quiet, but he may not simply enter.