tone down

Make less vivid, harsh, or violent; moderate. For example, That's a little too much rouge; I'd tone it down a bit, or Do you think I should tone down this letter of complaint? This idiom uses tone in the sense of "adjust the tone or quality of something," as does the antonym, tone up, meaning "brighten or strengthen." For example, These curtains will tone up the whole room, or This exercise is said to tone up the triceps. [Mid-1800s]

Definition: moderate
Antonyms: aggravate, increase, raise

See attenuation.

The verb has 2 meanings:

Meaning #1: deaden (a sound or noise), esp. by wrapping
  Synonyms: muffle, mute, dull, damp, dampen

Meaning #2: make less strong or intense; soften
  Synonyms: moderate, tame