I think your problem is that you have not added the "sounds.json" file under this directory:
<packname>/assets/minecraft/
The structure of this file should be like this example:
{
"entity.drowned.ambient": {
"subtitle": "drowning something",
"replace": false,
"sounds": [
{
"name": "custom/something",
"volume": 1,
"weight": 1,
"pitch": 1,
"attenuation_distance": 16,
"stream": false,
"preload": false
}
]
}
}
Basically, this example sounds.json file does the following:
"entity.drowned.ambient"
is the original sound to replace.
"subtitle": "drowning something"
the subtitle that shows if you have them enabled
"sounds": [
{
"name": "custom/something"
the path to your sound; in my example I would put my sound under
soundpack.zip/assets/minecraft/sounds/custom/something.ogg
(omit .ogg in json file)
"volume": 1,
"weight": 1,
"pitch": 1,
I have not messed with these, but you can if you like
"attenuation_distance": 16,
the distance from the source that you can hear the sound, in blocks
"stream": false,
"preload": false
I haven't messed with these either
If you want to make more changes to other sounds, you just continue the json structure. You can use https://jsoneditoronline.org to easily create, edit, and export .json documents in code or tree mode.