Possible Clues:

• Line up your peas with these, by the sound of them
• London lines
• American's lines - pronounced as signals for actors
• They're in line for billiards, by the sound of them
• 'Wait in them to make a cannon on the table, by the sound of them (6)'
• They're all in line for billiards, by the sound of them
• They're all lined up for letters, by the sound of them
• Line of waiting people or vehicles
• Reason for some holdups
• Audibly signals lots of waiters