Sweet means an attitude change. Humility, consideration for the other instruments, willingness to get into other beautiful things in life. It also means playing softly, not showing off or bragging. Letting the horn do your talking would be an excellent start.
Sweet is Wynton Marsalis, Manny Laureano, Matthias Höfs from German Brass, Chris Botti. Phil Smith plays very nicely, but I consider his playing to be very "forward", even on the cornet.