Waking The Silent One, and another puzzle later, requires an understanding of these musical machines.
Touch the Disciples and see if you recognise what they are doing.
They are directing you with their bodies, sort of like conductors, possibly more like arrangers. (Or interpretive dancers.)
Look at the Music Score you have previously collected from the Altar. (It's in the eye section of the inventory.)
Look at both their arms and their legs. Match the leg position to the arm position for each movement on the Music Score and press the corresponding button.
I am not sure if these are randomly generated, but in my game, the green Disciple did this. And these are the buttons I pressed.
Don't let Henry leave his instrument. Now click on the yellow Disciple and get Mum to use her instrument.
In my game, I already knew who appreciated The Silent One's music. It's possible you may not know this and so The Silent One may not want to help you.
Who would usually hear The Silent One summoning the Beast?
Spflornk of Pluto is a big fan. You may need to talk to him first, I'm not sure.
Got back to the Altar on Pluto to summon the Beast. Further hints will be for a terrifying concert.