This would be an issue with the mp3 itself, not a device issue. I have songs for all my texts and different messages, and they all play through. There is no setting to make them last longer.
Then this is obviously not a problem that is specific to the particular "You've Got Mail" tone. Where are you downloading these tones from? Did they all come from the same source? That could be your common denominator right there.
It is strange that only certain tones are not functioning properly on your device. Are all the file extensions the same? Please try to think of similarities between the situations surrounding the ringtones, if that makes sense.