Tones?

Hello,

What does tones mean in this sentence? A sound? a color?

Researchers had sleepers wear masks into which odors were pumped: shampoo and perfume, or carrion and rotting fish. The smells were accompanied with different tones.
The subjects continued to alter their breathing in response to the tones once they woke up, though they said they had no memory of smelling or hearing anything.

Thanks!