I used Helix Credits to buy a map of all Secrets of London locations. However I have collected some of them during gameplay, so cannot show you all of them, but at least most of the locations of Secrets of London - music boxes. You cannot find them with the Eagle Vision, only can hear the sound when you come closer. Not an easy task if you don't know exact location.
Hope it can help.
Have a nice game!
Assassin's Creed Syndicate Secrets of London locations.
AC Syndicate Secrets of London locations.

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