Dear Mr Albert, Hope you are doing great. I am writing in response to the complaint letter I received from you yesterday regarding the noise from my flat.
Dear Mr & Mrs Johnson, I am writing in response to your complaint letter about the noise from my apartment. I feel that I have to explain myself and humbly ask for your forgiveness.
Dear Mrs Barbara, I am very shocked to learn from your letter that the noise from my flat has been a source of disturbance for you and it has been spoiling your evening and causing distress. I am utterly regretful about that.