Android no sound in plugged-in loudspeaker?

I have a Samsung Android tablet with an external loudspeaker plugged into the sound jack socket. However, the tablet keeps speaking in Youtube throuh its own internal loudspeaker (quiet-murmurer I would call it).

I checked that the external loudspeaker works OK with a phone and the jack is firmly inserted and the volume control is at the maximum (anyway, the sound does come from the internal quiet-murmurer). Can this be an Android configuration problem ? What else can I check ?