Here is probably the biggest hurdle for this DIY description - all door chimes are probably more or less different. The three I have examined so far are all different, but they all have one thing in common - a LED which lights as the chime sounds. Here, the opto-coupler is put in parallell with the LED. The stuff is held in place with some hot-melt glue

Although the speaker can be disconnected to save power, it is somewhat a convenient feature - you get an audible feedback that the camera is triggered.