Under the MIT Licence, this resource pack allows for customization in modpacks including textures and sounds from 1.9.13 to 1.8.0 and some bonus textures and extra sounds.
To use different textures, you must copy and paste indivisual textures to your liking and adding it into the main texture files, replacing the currently using textures with your selective choice.
With sounds, you can modify it in the sounds.json files by changing the sound location under "sounds" under "entity.sound" example:

From:
 "ashyco.growl": {
        "category": "hostile",
        "replace": true,
        "sounds": [
            "srparasites:monster/adapted/shycoadapted/adaptedlongarmsliving1",
            "srparasites:monster/adapted/shycoadapted/adaptedlongarmsliving2",
            "srparasites:monster/adapted/shycoadapted/adaptedlongarmsliving3",
            "srparasites:monster/adapted/shycoadapted/adaptedlongarmsliving4"
        ]
    }
To:
 "ashyco.growl": {
        "category": "hostile",
        "replace": true,
        "sounds": [
            "srparasites:monster/adapted/shycoadapted/1.8.0-1.9.5/ashyco_growl"
        ]
    }

It is also important to remove or add a comma, before adding or replacing sounds to avoid errors.