The Custom Horns data pack includes many horns whose melodies are completely customizable! Simply craft a Horn Tuner block, place in any customizable horn (the ones with the golden name tags), then give it a toot! There are also mob horns which make the sounds of various mobs, and a reimplementation of the now cancelled Copper Horns!

Watch the whole playlist for the full tutorial!
https://www.youtube.com/playlist?list=PL_dqJNZ5Wmm3SorFPjY1BuGcJQo-qkTm6

If downloading directly through Planet Minecraft, you DO NOT have to extract the .ZIP folders. Simply place the downloads in your data pack folder and resource pack folder respectively. The versions downloadable through Google Drive must be extracted.

This data pack is incomplete. A final version with bug-fixes and optimizations is in the works. A standalone video tutorial will be uploaded in conjunction with the final update. For now, enjoy the WIP version!

Looking for something a little simpler? My Resource Pack Oxidize adds Copper Horns to the game with no data pack required! You can use it on any world!