Detect voltage in another circuit (isolated)


I would like to know when my OLD video doorbell is ringing.

Its something like this one: Amazon link

With a CRT flat screen.

I was thinking of detecting a voltage on the speaker (which play a ringtone when someone trigger at the door). Using a diode and a voltage divider to power an optocoupler and then retrieving the optocoupler state from an ESP8266 and send it over Wifi.

I would like to know if this could work or if you have any better ideas.

Thanks for reading.