Related Clues:

• Solid, as reasoning
• Without defect
• Now hear this!
• The obvious theme of this puzzle
• Like a good proof
• Audio
• Go off, that's all right
• Healthy, from what we hear
• Safe stretch of water, that's what we hear
• Noise, for example
• Valid, as an argument
• Channel something heard well
• All sides today physically fit
• Reliable output from loudspeaker
• Loudspeaker output