A device used on musical instruments to soften or muffle the sound. Common in orchestral and band contexts.
I practice with a mute on the trumpet.
弦楽器の弱音器は駒に取り付ける。
A string instrument mute is attached to the bridge.
Compound of 弱音 (weak sound) + 器 (device). The exact historical derivation is uncertain, but the term is a straightforward technical coinage.