There's a wide variety of high strength silver solders as well as brazing rods.

I'd suggest you look into a silver solder that will melt and join the metals at some temperature well under red hot so that you minimize the effect on the heat treating of the music wire.

Note that this does not mean using a silver bearing lead tin solder. While such solders are of higher strength than simple lead tin alloys they aren't really correctly called silver solder. The proper silver solders will not melt from contact with a soldering iron like the silver bearing lead tin ones will do.

And generally silver solders require a separate flux. Not the regular soldering flux either. You need the right stuff. I don't know about lowes or other more home style stores but any decent welding supply outfitters will have the silver solder in a variety of temperature ranges as well as the right flux. The flux I've used is a white paste in a little jar.

To use this solder I'd suggest you cut off a short length of the solder wire and bend it around the music wire and snug it up against the horn. Then apply the paste flux and heat. Doing it this way will ensure that when the heat is right the solder will flow. As soon as you see a nice fillet and it sucks down into the joint remove the heat to avoid overheating the music wire any more than absolutely required.