At different volumes, two tunes go by the book

I believe the answer is:

playfair

I'm a little stuck... Click here to teach me more about this clue!

'at different volumes two tunes' is the definition.
I can't judge whether this defines the answer.

'go by the book' is the wordplay.
'go' becomes 'play' (playing is a kind of going).
'by the book' becomes 'fair' (I am not sure about this - if you are sure you should believe this answer much more).
'play'+'fair'='PLAYFAIR'

Can you help me to learn more?

(Other definitions for playfair that I've seen before include "Decline to cheat" , "Act honestly" , "Don't cheat" .)

I've seen this clue in The Telegraph.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.