This addon was made by SDPhantom on the forum. Thanks to him 
The addon plays a sound when the life of your pet goes below 50% but also when it dies. Works for all classes that have pets like hunter, mage, death knights, etc.
If you wish to change when the sound is playing, open the core file and change 0.50 into something else. For example: 0.25 will play a sound when your pet has 25% life left. 0.80 means 80%...
You can also change the sounds in the Res folder and the core file its so easy I'm not gonna explain how to do it 