Duplo 10874 engine lacks sound without app

I have inherited this set which works perfectly apart from the fact that there is no sound from the engine. It clearly reads all the colours on the action bricks because, when used with the app, the little driver on the app blows the whistle and, through the app, I can hear the brakes and the water filling. This can’t be heard without the app. Also, the noise on pushing the little green button to start up makes no sound although I believe it’s supposed to. We’ve had it get taken apart to check but can’t see anything wrong.

Has anyone else had this problem? Any help much appreciated.