Possible Clues:

• Feature of piano leads to piano and forte being fluid
• Be restrained, take it easy - on bicycle?
• Piano's tone reducer
• Put one's foot down in a quiet sort of way?
• Tone down
• Tone down, as a statement
• Play down silly dope's fault — not acceptable for the top people
• It brings down the tone when depressed
• Play down – adopt self (anag.)
• Adopt self-regulating volume control