The singers simply sing the notes on the scale of the
keyboard, nothing new. The difference is, the note is
linked to a video of them singing each respective note.
To make it interesting, they could sing each note slightly
differently, vary the position of their lips for instance,
less vibrato for some notes, even comical or
character interpretations of the notes for others. Sing
note with a pirate face and an "Arrrrgh!" tone for
while another one sounding like a bird and making little
flapping wings with the hands. Basso guy might hit a
cliche body builder contest pose or something. No rules,
they'd just get
to have fun.
Then the person playing back the audio/video samples of
the various singers would be playing a virtual choir acting
out each note.