Possible Clues:

• Unit of frequency equal to one million cycles per second
• One million cycles per second
• Measure of cycles is very painful, reportedly?
• High frequency range, say, penetrating mum — is sore in the ears
• Dial units
• Unit on an FM dial
• Unit of frequency
• MHz in full
• Number on a dial
• A million cycles per second: excruciatingly painful, they say