However, I would like it to be silent, and got the impression on the forum that an optocoupler ( same as an optoisolators ? ) would be the better option.
Sounds like optoisolators would be a better option as they are silent and still offer electrical isolation. They are generally a 4 terminal device. Two of the pins are the input side of the opto and they are an internal led, so it would wire in series with your existing led. You may have to lower the series current limiting resistor so that the same led current flows with the now two series leds in each circuit. The two output pins of most simple optoisolators are just a simple npn transistor without the base connection. The emitter wires to your arduino ground pin and the collector can wire directly to a digital input pin. You must provide a pull-up resistor between the pin and the arduino's +5vdc pin, but that can be done by just enabling the internal pull-up for the pin with software. A low reading for the digital input pin means there is current flowing in the external led circuit and a high reading means there is no current flowing. Note that you can get optoisolator ICs that have one, two, or four independent isolators in one package if desired.http://en.wikipedia.org/wiki/Opto-isolator