In short I believe it (gameObj.GetComponent(Animal).speak()) should work, but it may not work with the non-typed versions of GetComponent like that. Here's what I've done that seems to work.

I've got a Component Damageable that extends Monobehaviour. From that I have 2 components that extend from Damageable. DroneHealth and ShipHealth.

DroneHealth and ShipHealth are attached to prefabs. In my collision test code I do

Damageable damagable = gotHit.transform.parent.GetComponent<Damageable>();

To get the component (the DroneHeralth or ShipHealth component that is, I never attach the Damageable component to anything).

I then call applyDamage, which has the signature public virtual void applyDamage(float amount, NetworkPlayer fromPlayer) in Damageable (and is overridden in the child classes) via RPC and the correct override in the subclass gets called (I was suprised this worked by RPC when I tested it, but it does. If it works that way I'm pretty sure it'll work for local calls).

Bare bones versions of the components

using UnityEngine;
using System.Collections;

public class Damageable : MonoBehaviour
{

    private float inVulnDuration = 0.0f;
    private float inVulnStart = 0.0f;

    protected bool damagable = true;
    public bool Damagable
    {
        get { return damagable; }
    }

    protected virtual void Update()
    {
        // if invulnerable 
        if (!damagable)
        {
            //check and see if we should become damageable yet
            if ((inVulnStart + inVulnDuration) < Time.timeSinceLevelLoad)
            {
                damagable = true;
            }
        }
    }

    [RPC]
    public virtual void applyDamage(float amount, NetworkPlayer fromPlayer)
    {
        Debug.Log(": " + "applyDamage in Damagable ");
    }
}



using UnityEngine;
using System.Collections;

public class ShipHealth : Damageable
{
    protected override void Update()
    {
        base.Update();
        // if invulnerable 
        if (!damagable)
        {
            // code to make ship blink when invulnerable
        }
    }

    [RPC]
    public override void applyDamage(float amount, NetworkPlayer fromPlayer)
    {
            Debug.Log(": " +"applyDamage in shipHelath");
    }
}

Sounds like you should be using SendMessage.

gameObj.SendMessage("Speak");

In that case it doesn't matter what the component is called.