Luckily, I had a backup of Hexxit Modpack. You can download it and replace "resource folder" in your Hexxit Modpack, all sound will return. Sorry for my english =.="
Here: https://www.mediafire.com/file/bb957eftx7a8xm5/Hexxit-Sound.zip/file
Unzip and replace it in: %appdata%\.technic\modpacks\hexxit\