>>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.
>
>I think you might be right about the air restriction. I initially didn't cut down the cork, but it sounded pretty loud still, so I cut more. Think I'll look into a good mute that will mellow the tone and worry less about the volume. It was just frustrating for her because she thought she was not playing well at home and couldn't understand why it sounded fine at practice.

Is she receiving some sort of professional lessons? If so, shouldn't the instructor be able to help her out? Once upon a time I probably would have known the answer, but my trumpeting knowledge has long been overwritten.