I was always interested when my parents watched Lawrence Welk, to see the brass section use mutes. It was many years before I really understood how they worked, but I knew they changed the sound of the instrument.
I've always enjoyed watching big bands and how they change the sounds of their instruments to conform to the needs of the music. I have always wondered, though, if a trumpet or trombone player really notices the mute in place? Other than the changed sound, does it change the technique, or the way the musician breathes or similar?