Tutorial How to Change the sounds in Minecraft
// by Sencie https://www.planetminecraft.com/texture-pack/old-sounds-4911323/

How to change the sounds?
1. Extract sounds.json from the pack assets/minecraft/ and make a copy in case you have a problem.
2. Open sounds.json with a text editor such as Notepad (Windows) or Apple TextEdit (Mac OS X).
3. Find the item you want to modify and change the lines under it.
4. If your pack does not work, your sounds.json is broken. Most problems are typo and missing comma. Use www.jsonlint.com to locate the problem in your code.

How to make a sound silent?
For instance, we silence the music in the menu like this:
  "music.menu": {
    "replace": true,
    "sounds": []
  },

How to change a sound with an existing one?
For instance, we replace step/grass1-grass6 by dig/grass1-grass4 like this:
  "block.grass.step": {
    "replace": true,
    "sounds": [
      "dig/grass1",
      "dig/grass2",
      "dig/grass3",
      "dig/grass4"
    ],
    "subtitle": "subtitles.block.generic.footsteps"
  },

How to add a new sound? 
1. In this example, we added a new sound (hurtclassic.ogg) in the pack assets/minecraft/sounds/damage/hurtclassic.ogg
2. Edit the sounds.json with:
  "entity.player.hurt": {
    "replace": true,
    "sounds": ["damage/hurtclassic"],
    "subtitle": "subtitles.entity.player.hurt"
  },

How to restore a sound from default Minecraft?
First method: Open sounds.json and delete all the item lines. For instance, to restore the music in the menu, delete all of the following:
  "music.menu": {
    "replace": true,
    "sounds": []
  },
Second method: 
1. Open unmodified_sounds.json in the pack assets/minecraft
2. Find the item you want to restore and copy its lines of text.
3. Extract and open sounds.json and paste the copied text under the same item.

Where are the original sounds both for mining and steps?
STONE: dig/stone1-stone4
GRASS: dig/grass1-grass4
DIRT/GRAVEL: dig/gravel1-gravel4
WOOD: dig/wood1-wood4
SAND: dig/sand1-sand4