A Dictionary of Music and Musicians/Muta
MUTA (Italian), i.e. change. A word often seen attached to Horn parts—'muta in Es,' 'muta in B,' etc., meaning simply 'change to E♭ or B♭,' etc.; that is, take off the crook in which you are playing and put on that which will make the horn sound in E♭ or B♭.
[ G. ]