Mix up a bit of commercial hide glue (Lowe's or Home Depot) with a little water. Pour it into the horn from the spout, swirl the horn around and then set the horn up on the base plug so the glue flows toward the leak. Leave it for a few days to dry thoroughly. Don't worry about any that leaks out onto the base plug as hide glue is water soluble and can be cleaned off easily.
I've thinned out Titebond with water, poured it in the spout hole, stand it up for awhile and then blow down the hole. The glue mix will come out the air leaks, wipe it off with a damp rag and let it dry. May have to do it a couple times. Tim C.
Wood glue or varnish - put in with a paper funnel and straw. Then strategically sit upright with a fan blowing over the open spout. Will take a long time to dry. Keep in mind, that if it’s a translucent horn, whatever you put inside will be visible from the outside. Better to work some warm beeswax on the outside down into the seam with those.
Thin CA glue from the outside where the leak is. Wipe off any excess with CA glue solvent. Much quicker than going from the inside and you will never see it.