soft-pedal

Short definition from Wiktionary

Verb

1. 1
   To reduce the volume of (music, a sound, etc.).
2. 2
   To reduce the force or impact of (something); to damp, to mute; especially, to minimize the less desirable aspects of (something); to play down, to tone down.
3. 3
   To attempt to persuade someone about (something) through understatement, so that the listener accepts the good points as obvious.
4. 4
   Chiefly followed by on: to act in a less assertive or forceful manner.

Noun

1. 1
   The leftmost foot-pedal on most pianos which, when pushed, dampens the strings and so softens the note played.