A mute is a device attached to a musical instrument which changes the instrument's tone quality (timbre) or lowers its volume. Mutes are commonly used on string and brass instruments, especially the trumpet and trombone, and are occasionally used on woodwinds. Their effect is mostly intended for artistic use, but they can also allow players to practice discreetly. Muting can also be done by hand, as in the case of palm muting a guitar or grasping a triangle to dampen its sound.

Mutes on brass instruments are typically inserted into the flared end of the instrument (bell). They can also be held in front of or clipped onto the bell. Of brass mutes, the "straight mute" is the most common and is frequently used in classical and jazz music, but a wide variety are available. On string instruments of the violin family, mutes are usually attached to the bridge, the piece of wood that supports the strings. Palm muting a guitar involves placing the side of the hand across the strings, and some of them have physical mutes which produce a similar effect. Pianos have a soft pedal and occasionally a practice pedal, which both decrease the instrument's volume.

According to The New Grove Dictionary of Music and Musicians, a mute is a "device used on a musical instrument to modify its timbre by reducing the intensity of certain partials and amplifying others". More generally, it refers to "any of various devices used to muffle or soften the tone of an instrument". The act of using a mute is called "muting". Brass mutes are occasionally referred to as "dampers", but "damping" usually means reducing or deadening the sound after it has been played.

Mutes can be used artistically and for practical purposes. They are used in several genres of music: violin mutes have been used in classical music since at least the seventeenth century, and many types of brass mutes are used in jazz. A common misconception is that muting has the sole purpose of decreasing volume, but this is only true of practice mutes; string instruments in particular can easily play softly without additional equipment.

Musical notation directing players to mute and unmute their instruments varies. The type of mute and when to add and remove is specified in text above the music; open is often used in music for brass to indicate the subsequent passage should be played without a mute. In classical music, the phrase con sordino or con sordini (Italian: with mute, abbreviated con sord.), directs players to use a straight mute on brass instruments, and mount the mute on string instruments. The corresponding senza sordino indicates removing the mute. Synonyms include avec sourdine and sans sourdine (French); mit Dämpfer and ohne Dämpfer (German). Ample time must be given to allow players to add and remove the mute, though some mutes can be engaged and disengaged quickly.

For hand muting that changes over time, o is used above the music for open (unmuted) and + for closed (muted). For dampening on percussion or harp, the étouffé symbol (resembling a coda symbol) or a diamond-shaped notehead is used.

Mutes are widely used on brass instruments to alter their timbre. They are often directly inserted into the instrument's bell, but can also be clipped or held onto the end of the bell. Mutes of various types are available in many sizes for all brass instruments, including the tuba; trumpet and trombone players have the widest selection of mutes. They are made of a variety of materials, including fiber, plastic, cardboard, and metal (usually aluminum, brass, or copper). In general, mutes soften the lower frequencies of the sound because they nearly close the bell, but accentuate higher ones due to resonances within the mute.

Stoppers for natural trumpets (the predecessor of the modern valved trumpet) were found in the tomb of King Tutankhamun dating to around 1300 BC, but they were most likely for protecting the instrument from moisture or damage during transport. The earliest known mention of trumpet mutes dates to a 1511 account of a carnival in Florence. Claudio Monteverdi's 1607 opera L'Orfeo opens with a muted trumpet ensemble, which musicologist Wolfgang Osthoff suggests is because the piece was first performed in a small, intimate chamber. These early mutes, known as Baroque mutes, were fashioned from wood and had a hole in the center to permit airflow. They raised the pitch by a semitone or more when inserted, which could be corrected by adding a piece of tubing of appropriate length, known as a crook. Besides musical use, Baroque mutes were used for secret military retreat, funerals, and practice.