Our client is a Parkinson's sufferer and found the AC buzzer in his flat intercom too loud and sharp, and needed something which didn't make him jump every time the doorbell was pressed.
The inspiration for the solution came from two sources:
1. The progressive ring tone the client had already found for his mobile phone
2. Commercially available sound recording doorbells which allow you to store your favourite tune
The AC buzzer was replaced with a small DC rectifier and voltage regulator circuit, wired to a self contained sound recording module purchased from an electronics website. Pressing the doorbell energises the sound recording module and plays the recording stored in its non-volatile memory through a small loudspeaker.
Buttons and microphone to allow recording and playback of a new sound were incorporated into the case of the intercom phone, and are activated with a plug-in battery pack.
A full description of the operation to accompany the diagram is available in Word format here.