What a sum of experience and knowledge are brought together in this forum!

Mario D, thank you for the idea of using the mod wheel: I didn't know it allows to change the volume of the ARIA Player! But here, this is not the solution to my problem.

For my case, the winner is Andrew! Indeed, to have the sound of an instrument via the ARIA Player, you must not only, in the Band in a Box mixer, select the ARIA Player as plugin for the chosen track, but then, in the window of the ARIA Player, indicate to which MIDI channel the selected instrument should be directed.

And this is not obvious, because the MIDI channel choice zone is not clearly indicated in the ARIA Player: it is in fact the small gray number on the far right of the line under the name of the instrument and you have to click on it to bring up the list of MIDI channels...

Anyway, problem solved. Thank you all!

Lazy Bird