You can't change the eating animation sound, it's a limitation of the game.
But you can add a custom sound after you eat the item or right after you start right clicking it to use it.
info:
namespace: example
items:
example_food:
enabled: true
display_name: Example food
resource:
generate: true
textures:
- item/example_food
material: APPLE
events:
eat:
feed:
amount: 1
play_sound:
name: ambient.cave
volume: 1
pitch: 1
interact:
right:
play_sound:
name: block.barrel.open
volume: 1
pitch: 1
Don't forget to use the official editor to easily edit files.
https://itemsadder.devs.beer/plugin-usage/files-editor