The sound boards are large plate bricks that emit the piano sound. The more there are, the louder the volume becomes. This one has 4, so each note has the sound of 4.
Hm... In the version of the build you posted, all the keys have the notes playing through the keys themselves, not the "soundboards". Might wanna check the version you uploaded.
Yeah, really that is just so I can easily put the sounds onto the piano by just starting from 1 to 60 left to right. My convenience really, its not intended to be used for another build but you could since its not too hard to order the sounds from scratch. What I really should have done is named the keys more appropriately as bricks, so it is easier to make rolls. I need to post the diagram that shows how to easily make rolls. :U
Actually, the more intuitive route for musicians (the most likely group to make rolls) would have been to just use the note names. That way, they can associate a C with a C, not have to convert the numbers to the right notes (white1, 8, 15 are C; 2, 9, 16 are D; etc.)
Also, if I was the one making this soundpack (I hate using phrases like that...) I would have made it as easy as possible to pick out the right notes if somebody wanted to make custom songs. I would have picked a nice prefix like LateralPiano_ or LPiano_ and then added the note name+octave, so for example you'd have LPiano_C2.wav, LPiano_C3.wav, etc. etc. Your choice though. You're the soundpack maker here, not I.
Lastly, I apologize if I sound negative in these posts. I am just trying to give you criticism which you might be able to use to improve your already-quite-awesome build.