>Ok, all you musicians out there. A mute should not change how the tone of a note sounds, just how loud. Right? We have a promising Alison Balsom here and when we put the mute in her trumpet, the notes tend to break and are not clean. I tried it as well with the same result. Take the mute out and they are good. It's a Humes & Berg student mute, if that matters. The corks are filed down much, but seems to be plenty of room for air to get through. I think the trumpet is a King (she's practicing now). Any ideas?

Different mutes will certainly change the tone of the note. That's their real purpose, not so much to change the decibel level (though they do that too). You want a tinny sound, you get one kind of mute, you want a hollower sound, another kind of mute. Listen to Miles Davis (he loved his mutes) and you will hear the difference in sound between muted and unmuted.

The mute shouldn't cause the sound to crack. It should still be pure, but the sound itself will definitely change.

The corks can be bought individually, and if they are too flat, then yes, change them. Too little air getting through can be an issue.