Is there a way I can add more variety of sounds to a mob?

Yes, it's definitely possible, though requires some file management.

1. In your resource pack, navigate to the assets/minecraft folder.
2. Create a file called sounds.json. You will need a text editor that understands json files.
3. Navigate to the indexes folder, and find the version you're creating a resource pack for.
4. Ctrl+F for "sounds"; you should find a long hexadecimal string. Note this down.
5. Now, head to your .minecraft folder (Open the Run program, run %appdata%/.minecraft). Search for the hexadecimal in that folder.
6. Copy the contents of that file to your sounds.json file.
7. There should be something like the following:

"entity.villager.hurt": {
"sounds": [
"mob/villager/hit1",
"mob/villager/hit2",
"mob/villager/hit3"
}

8. It's now simple to add sounds. Simply copypaste "mob/villager/hitN", increasing N by 1 until you reach the number of sounds you want.
9. Then, in the sounds folder for your resource pack, you can name your sounds Hit4.ogg, Hit5.ogg, etc.

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   This should work for every sound file in the game. For a list of sound files, check out this page on the Minecraft Wiki.