no buzz, no sound. The lips are the sound generator. There is nothing else in the system that can get the air to vibrate. The difference between buzzing in free air, on the mouthpiece or into the horn is the resonance that you get by adding hardware. The resonance of the instrument supports the buzz at a particular frequency, making it easier to hold out a note steadily with constant sound quality.
The horn does not make the lips vibrate, only the player can induce that. The instrument only tries to make the lips vibrate at specific frequencies related to the length of the horn.